7.2-Section Formula

7.2-Section Formula Important Formulae

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Grade 10 → Math → Coordinate Geometry → 7.2-Section Formula

After successful completion of this topic, you should be able to:

  • Apply and derive section formula in order to divide the line segment in a given ratio
  • Apply distance and section formula in order to determine the vertices/diagonals/mid points of given geometrical shapes.

The section formula is a key concept in coordinate geometry used to determine the coordinates of a point that divides a line segment into a given ratio. This formula is particularly useful when working with points in a Cartesian plane.

If we have two points, $A(x_1, y_1)$ and $B(x_2, y_2)$, and a point $P$ divides the line segment $AB$ in the ratio $m:n$, then the coordinates of point $P(x, y)$ can be calculated using the section formula.

The formula for the coordinates of point $P$ is given by:

$P\left(x, y\right) = \left(\frac{{mx_2 + nx_1}}{{m + n}}, \frac{{my_2 + ny_1}}{{m + n}}\right)$

Here:

  • $P(x, y)$ = Coordinates of the point dividing the segment
  • $A(x_1, y_1)$ = Coordinates of the first point
  • $B(x_2, y_2)$ = Coordinates of the second point
  • $m$ = Ratio of division (part from $B$)
  • $n$ = Ratio of division (part from $A$)

To illustrate the section formula, let’s consider an example. Suppose we have two points:

Point $A(2, 3)$ and Point $B(8, 5)$, and we want to find the coordinates of point $P$ that divides the segment $AB$ in the ratio $3:2$.

Using the section formula:

$P\left(x, y\right) = \left(\frac{{3 \cdot 8 + 2 \cdot 2}}{{3 + 2}}, \frac{{3 \cdot 5 + 2 \cdot 3}}{{3 + 2}}\right)$

Calculating the $x$-coordinate:

$x = \frac{{24 + 4}}{{5}} = \frac{{28}}{{5}} = 5.6$

Now, calculating the $y$-coordinate:

$y = \frac{{15 + 6}}{{5}} = \frac{{21}}{{5}} = 4.2$

Therefore, the coordinates of point $P$ are $P\left(5.6, 4.2\right)$.

In the case where point $P$ divides the segment externally, the section formula is slightly modified. If point $P$ divides the line segment $AB$ externally in the ratio $m:n$, the coordinates can be calculated as:

$P\left(x, y\right) = \left(\frac{{mx_2 - nx_1}}{{m - n}}, \frac{{my_2 - ny_1}}{{m - n}}\right)$

For example, if we take the same points $A(2, 3)$ and $B(8, 5)$ and consider point $P$ dividing the segment externally in the ratio $3:2$, we can apply the external section formula:

$P\left(x, y\right) = \left(\frac{{3 \cdot 8 - 2 \cdot 2}}{{3 - 2}}, \frac{{3 \cdot 5 - 2 \cdot 3}}{{3 - 2}}\right)$

Calculating the $x$-coordinate:

$x = \frac{{24 - 4}}{{1}} = 20$

Calculating the $y$-coordinate:

$y = \frac{{15 - 6}}{{1}} = 9$

Thus, the coordinates of point $P$ in the external division case are $P(20, 9)$.

The section formula is extensively used in coordinate geometry for finding points that partition line segments, making it essential for various geometric constructions and proofs.

Find the coordinates of the point which divides the join of (–1, 7) and (4, –3) in the ratio 2 : 3.

Solution:

Question: Find the coordinates of the point which divides the join of (–1, 7) and (4, –3) in the ratio 2 : 3.
Let the coordinates of the point dividing the line segment joining $(x_1, y_1)$ and $(x_2, y_2)$ in the ratio $m : n$ be $(x, y)$.

The formula for the coordinates of the point dividing the line segment in the ratio $m : n$ is:

$ x = \frac{m x_2 + n x_1}{m + n} $

$ y = \frac{m y_2 + n y_1}{m + n} $

Here, the coordinates of the first point are $(x_1, y_1) = (-1, 7)$, the coordinates of the second point are $(x_2, y_2) = (4, -3)$, and the ratio is $2 : 3$.

Using the formula for $x$:

$ x = \frac{2 \times 4 + 3 \times (-1)}{2 + 3} $

$ x = \frac{8 - 3}{5} $

$ x = \frac{5}{5} = 1 $

Now, using the formula for $y$:

$ y = \frac{2 \times (-3) + 3 \times 7}{2 + 3} $

$ y = \frac{-6 + 21}{5} $

$ y = \frac{15}{5} = 3 $

Therefore, the coordinates of the point are $(1, 3)$.

Find the coordinates of the points of trisection of the line segment joining (4, –1) and (–2, –3).

Solution:

Find the coordinates of the points of trisection of the line segment joining (4, –1) and (–2, –3).

Let the points of trisection divide the line segment joining A(4, –1) and B(–2, –3) into three equal parts. Let the points of trisection be P and Q, where P divides the line segment in the ratio 1:2 and Q divides the line segment in the ratio 2:1.

Using the section formula, the coordinates of a point dividing a line segment in the ratio $m:n$ are given by:

$\left( \dfrac{mx_2 + nx_1}{m+n}, \dfrac{my_2 + ny_1}{m+n} \right)$

For point P, dividing the line segment in the ratio 1:2:

Let P be the point dividing AB in the ratio 1:2. So, $m = 1$ and $n = 2$.

The coordinates of P are:

$P = \left( \dfrac{1 \times (-2) + 2 \times 4}{1+2}, \dfrac{1 \times (-3) + 2 \times (-1)}{1+2} \right)$

$P = \left( \dfrac{-2 + 8}{3}, \dfrac{-3 - 2}{3} \right)$

$P = \left( \dfrac{6}{3}, \dfrac{-5}{3} \right)$

$P = (2, -\dfrac{5}{3})$

For point Q, dividing the line segment in the ratio 2:1:

Let Q be the point dividing AB in the ratio 2:1. So, $m = 2$ and $n = 1$.

The coordinates of Q are:

$Q = \left( \dfrac{2 \times (-2) + 1 \times 4}{2+1}, \dfrac{2 \times (-3) + 1 \times (-1)}{2+1} \right)$

$Q = \left( \dfrac{-4 + 4}{3}, \dfrac{-6 - 1}{3} \right)$

$Q = \left( \dfrac{0}{3}, \dfrac{-7}{3} \right)$

$Q = (0, -\dfrac{7}{3})$

To conduct Sports Day activities, in your rectangular shaped school ground ABCD, lines have been drawn with chalk powder at a distance of 1m each. 100 flower pots have been placed at a distance of 1m from each other along AD, as shown in Fig.7.12. Niharika runs 14 ththe distance AD on the 2nd line and posts a green flag. Preet runs 15 th the distance AD on the eighth line and posts a red flag. What is the distance between both the flags? If Rashmi has to post a blue flag exactly halfway between the line segment joining the two flags, where should she post her flag?

Solution:

Distance Between Two Flags and Midpoint Calculation

Let the length of side AD of the rectangular school ground ABCD be represented by $l$. The distance between two points along AD is measured in meters, with each line spaced 1 meter apart.

The positions of the flags are determined as follows:

  • Niharika posts a green flag on the 2nd line, at $ \frac{1}{4} \times l $ meters along AD.
  • Preet posts a red flag on the 8th line, at $ \frac{1}{5} \times l $ meters along AD.

To find the distance between the two flags, we calculate the difference in their positions along the AD line:

Distance between the two flags = $ \left| \frac{1}{4} \times l - \frac{1}{5} \times l \right| $

Now, to calculate Rashmi's position for posting the blue flag, we need to find the midpoint between the two flags. The midpoint is the average of the positions of the green and red flags:

Midpoint = $ \frac{ \left( \frac{1}{4} \times l \right) + \left( \frac{1}{5} \times l \right) }{2} $

Simplifying the expression for the midpoint:

Midpoint = $ \frac{ \left( \frac{5}{20} \times l \right) + \left( \frac{4}{20} \times l \right) }{2} $

Midpoint = $ \frac{ \frac{9}{20} \times l }{2} $

Midpoint = $ \frac{9}{40} \times l $

Rashmi should post the blue flag at a distance of $ \frac{9}{40} \times l $ meters along AD.

Find the ratio in which the line segment joining the points (– 3, 10) and (6, – 8) is divided by (– 1, 6).

Solution:

Question: Find the ratio in which the line segment joining the points (– 3, 10) and (6, – 8) is divided by (– 1, 6).
Solution:
Let the required ratio be $k : 1$. Using the section formula, the coordinates of the point dividing the line segment in the ratio $k : 1$ are given by: $P\left(\frac{kx_2 + x_1}{k + 1}, \frac{ky_2 + y_1}{k + 1}\right)$ Here, the coordinates of the points are: $A(x_1, y_1) = (-3, 10)$ $B(x_2, y_2) = (6, -8)$ $P(x, y) = (-1, 6)$ Substitute the values in the section formula: $-1 = \frac{k(6) + (-3)}{k + 1}$ and $6 = \frac{k(-8) + 10}{k + 1}$. Now, solve the first equation: $-1 = \frac{6k - 3}{k + 1}$ Multiplying both sides by $(k + 1)$: $-1(k + 1) = 6k - 3$ $-k - 1 = 6k - 3$ Simplifying: $-k - 1 = 6k - 3$ $-1 + 3 = 6k + k$ $2 = 7k$ $k = \frac{2}{7}$ Thus, the ratio in which the line segment is divided is $2:7$.

Find the ratio in which the line segment joining A(1, – 5) and B(– 4, 5) is divided by the x-axis. Also find the coordinates of the point of division.

Solution:

Question: Find the ratio in which the line segment joining A(1, – 5) and B(– 4, 5) is divided by the x-axis. Also find the coordinates of the point of division.

Let the point of division be P(x, 0). The coordinates of A are (1, –5) and the coordinates of B are (–4, 5).

Since the point P lies on the x-axis, its y-coordinate is 0. Let the ratio in which P divides AB be $m : n$.

Using the section formula, the coordinates of P dividing AB in the ratio $m : n$ are given by:

The x-coordinate of P: $x = \frac{m \cdot x_2 + n \cdot x_1}{m + n}$

The y-coordinate of P: $y = \frac{m \cdot y_2 + n \cdot y_1}{m + n}$

Substituting the values for the coordinates of A and B, we have:

For the x-coordinate of P:

$x = \frac{m \cdot (-4) + n \cdot 1}{m + n}$

For the y-coordinate of P:

$0 = \frac{m \cdot 5 + n \cdot (-5)}{m + n}$

Solving the equation for the y-coordinate:

$0 = \frac{5m - 5n}{m + n}$

This simplifies to $5m = 5n$, which gives $m = n$.

Now substitute $m = n$ into the equation for the x-coordinate of P:

$x = \frac{m \cdot (-4) + m \cdot 1}{m + m}$

$x = \frac{-4m + m}{2m}$

$x = \frac{-3m}{2m}$

$x = -\frac{3}{2}$

Therefore, the point P divides the line segment in the ratio $1 : 1$ and the coordinates of the point of division are $\left(-\frac{3}{2}, 0\right)$.

If (1, 2), (4, y), (x, 6) and (3, 5) are the vertices of a parallelogram taken in order, find x and y.

Solution:

Given vertices of a parallelogram: (1, 2), (4, y), (x, 6), (3, 5)

We know that the diagonals of a parallelogram bisect each other. So, the midpoint of diagonal AC is the same as the midpoint of diagonal BD.

Let the vertices be labeled as follows: A(1, 2), B(4, y), C(x, 6), and D(3, 5).

The midpoint of diagonal AC is the midpoint of points A(1, 2) and C(x, 6), and the midpoint of diagonal BD is the midpoint of points B(4, y) and D(3, 5).

Midpoint of AC:
$ \left( \frac{1 + x}{2}, \frac{2 + 6}{2} \right) = \left( \frac{1 + x}{2}, 4 \right)$

Midpoint of BD:
$ \left( \frac{4 + 3}{2}, \frac{y + 5}{2} \right) = \left( \frac{7}{2}, \frac{y + 5}{2} \right)$

Since the midpoints of diagonals bisect each other, we equate the corresponding coordinates.

Equating the x-coordinates:
$ \frac{1 + x}{2} = \frac{7}{2} $
$ 1 + x = 7 $
$ x = 6 $

Equating the y-coordinates:
$ 4 = \frac{y + 5}{2} $
$ 8 = y + 5 $
$ y = 3 $

Therefore, the values of x and y are:

$ x = 6 $ and $ y = 3 $

Find the coordinates of a point A, where AB is the diameter of a circle whose centre is (2,– 3) and B is (1,4).

Solution:

Solution:
Let the coordinates of point A be $(x_1, y_1)$ and the coordinates of point B are given as $(1, 4)$. The centre of the circle is given as $(2, -3)$. Since AB is the diameter of the circle, the midpoint of AB is the centre of the circle. The midpoint formula is: $Midpoint = \left(\frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2}\right)$ Here, the midpoint of AB is the centre $(2, -3)$, and point B is $(1, 4)$. Let the coordinates of point A be $(x_1, y_1)$. Using the midpoint formula: $\left(\frac{x_1 + 1}{2}, \frac{y_1 + 4}{2}\right) = (2, -3)$ Now, equating the x-coordinates and y-coordinates: $\frac{x_1 + 1}{2} = 2$ and $\frac{y_1 + 4}{2} = -3$ Solving the first equation: $\frac{x_1 + 1}{2} = 2$ Multiply both sides by 2: $x_1 + 1 = 4$ Subtract 1 from both sides: $x_1 = 3$ Solving the second equation: $\frac{y_1 + 4}{2} = -3$ Multiply both sides by 2: $y_1 + 4 = -6$ Subtract 4 from both sides: $y_1 = -10$ Therefore, the coordinates of point A are $(3, -10)$.

If A and B are (– 2, – 2) and (2, – 4), respectively, find the coordinates of P such that AP = $\dfrac{3}{7}$ AB and P lies on the line segment AB.

Solution:

Coordinates of P such that AP = $\dfrac{3}{7}$ AB
Let the coordinates of A be $(x_1, y_1) = (-2, -2)$ and the coordinates of B be $(x_2, y_2) = (2, -4)$. The formula to find the coordinates of P dividing the line segment AB in the ratio m:n is: $P = \left( \dfrac{n x_1 + m x_2}{m + n}, \dfrac{n y_1 + m y_2}{m + n} \right)$ Here, $AP = \dfrac{3}{7} AB$, so the ratio of AP to PB is 3:4 (since $AP + PB = AB$). Thus, $m = 3$ and $n = 4$. Substitute the values of $x_1, y_1, x_2, y_2, m,$ and $n$ into the formula: $P = \left( \dfrac{4(-2) + 3(2)}{3 + 4}, \dfrac{4(-2) + 3(-4)}{3 + 4} \right)$ Simplify: $P = \left( \dfrac{-8 + 6}{7}, \dfrac{-8 - 12}{7} \right)$ $P = \left( \dfrac{-2}{7}, \dfrac{-20}{7} \right)$ Thus, the coordinates of P are $\left( \dfrac{-2}{7}, \dfrac{-20}{7} \right)$.

Find the coordinates of the points which divide the line segment joining A(– 2, 2) and B(2, 8) into four equal parts.

Solution:

Find the coordinates of the points which divide the line segment joining A(– 2, 2) and B(2, 8) into four equal parts.

The coordinates of the points dividing the line segment joining two points in a given ratio can be found using the section formula. The section formula is given by:

For a point dividing the line segment joining points $A(x_1, y_1)$ and $B(x_2, y_2)$ in the ratio $m:n$, the coordinates of the point $(x, y)$ are given by:

$x = \dfrac{m x_2 + n x_1}{m + n}$

$y = \dfrac{m y_2 + n y_1}{m + n}$

Here, the points $A(– 2, 2)$ and $B(2, 8)$ are given. We need to find the coordinates of the points dividing the segment into four equal parts. This means the ratio will be 1:3, 2:2, and 3:1, and the point dividing the segment in each of these ratios will give the required coordinates.

Step 1: Coordinates of the point dividing the line in the ratio 1:3

Using the section formula for the ratio 1:3, we have:

$x = \dfrac{1 \cdot 2 + 3 \cdot (-2)}{1 + 3} = \dfrac{2 - 6}{4} = \dfrac{-4}{4} = -1$

$y = \dfrac{1 \cdot 8 + 3 \cdot 2}{1 + 3} = \dfrac{8 + 6}{4} = \dfrac{14}{4} = 3.5$

So, the coordinates of the point dividing the segment in the ratio 1:3 are $(-1, 3.5)$.

Step 2: Coordinates of the point dividing the line in the ratio 1:1

For the ratio 1:1, the midpoint is found:

$x = \dfrac{1 \cdot 2 + 1 \cdot (-2)}{1 + 1} = \dfrac{2 - 2}{2} = 0$

$y = \dfrac{1 \cdot 8 + 1 \cdot 2}{1 + 1} = \dfrac{8 + 2}{2} = \dfrac{10}{2} = 5$

So, the coordinates of the midpoint are $(0, 5)$.

Step 3: Coordinates of the point dividing the line in the ratio 3:1

Using the section formula for the ratio 3:1, we have:

$x = \dfrac{3 \cdot 2 + 1 \cdot (-2)}{3 + 1} = \dfrac{6 - 2}{4} = \dfrac{4}{4} = 1$

$y = \dfrac{3 \cdot 8 + 1 \cdot 2}{3 + 1} = \dfrac{24 + 2}{4} = \dfrac{26}{4} = 6.5$

So, the coordinates of the point dividing the segment in the ratio 3:1 are $(1, 6.5)$.

Step 4: Coordinates of the point dividing the line in the ratio 1:3 (other direction)

The coordinates of the last point dividing the line in the ratio 3:1 are the same as in Step 3:

So, the coordinates are $(1, 6.5)$.

Find the area of a rhombus if its vertices are (3, 0), (4, 5), (– 1, 4) and (– 2, – 1) taken in order. [Hint : Area of a rhombus = $\dfrac{1}{2}$ (product of its diagonals)]

Solution:

Find the Area of a Rhombus

Given vertices of the rhombus are: (3, 0), (4, 5), (–1, 4), and (–2, –1).

To find the area of a rhombus, we use the formula:

Area = $ \frac{1}{2} \times d_1 \times d_2 $

Where $d_1$ and $d_2$ are the lengths of the diagonals of the rhombus.

First, calculate the length of the diagonals:

The diagonals are the line segments joining opposite vertices.

Diagonal 1: From (3, 0) to (–1, 4)

Length of Diagonal 1, $d_1 = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}$

$d_1 = \sqrt{((-1) - 3)^2 + (4 - 0)^2} = \sqrt{(-4)^2 + 4^2} = \sqrt{16 + 16} = \sqrt{32} = 4\sqrt{2}$

Diagonal 2: From (4, 5) to (–2, –1)

Length of Diagonal 2, $d_2 = \sqrt{(x_2 - x_1)^2 + (y_2 - y_1)^2}$

$d_2 = \sqrt{((-2) - 4)^2 + ((-1) - 5)^2} = \sqrt{(-6)^2 + (-6)^2} = \sqrt{36 + 36} = \sqrt{72} = 6\sqrt{2}$

Now, substitute the values of $d_1$ and $d_2$ into the area formula:

Area = $ \frac{1}{2} \times 4\sqrt{2} \times 6\sqrt{2} $

Area = $ \frac{1}{2} \times 24 \times 2 $

Area = $ \frac{1}{2} \times 48 $

Area = 24 square units