3.2-Substitution Method

3.2-Substitution Method Important Formulae

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Grade 10 → Math → Pair of Equations in Two Variables → 3.2-Substitution Method

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

  • Use the most appropriate algebraic method in order to solve the given pair of linear equations.

The substitution method is a powerful technique used to solve a pair of linear equations in two variables. This method involves expressing one variable in terms of the other and then substituting it into the second equation. This approach simplifies the equations, making it easier to find the values of the variables.

1. Steps to Solve Using the Substitution Method

To solve a pair of equations using the substitution method, follow these steps:

  • Step 1: Start with the given pair of equations. For example:
  • $$\begin{align*} (1) \quad 2x + 3y &= 12 \\ (2) \quad x - y &= 1 \end{align*}$$

  • Step 2: Solve one of the equations for one variable. Here, we can solve equation (2) for $x$:
  • $$x = y + 1$$

  • Step 3: Substitute this expression into the other equation (1):
  • $$2(y + 1) + 3y = 12$$

  • Step 4: Simplify and solve for the remaining variable:
  • $$2y + 2 + 3y = 12$$

    $$5y + 2 = 12$$

    $$5y = 10$$

    $$y = 2$$

  • Step 5: Substitute the value of $y$ back into the equation for $x$:
  • $$x = 2 + 1 = 3$$

  • Step 6: State the solution as an ordered pair $(x, y)$:
  • $$\text{Solution: } (3, 2)$$

2. Example Problem

Let's look at another example:

Consider the equations:

$$\begin{align*} (1) \quad 3x + 4y &= 24 \\ (2) \quad x - 2y &= 1 \end{align*}$$

Using the substitution method, we can solve this step by step:

  • From equation (2), express $x$ in terms of $y$:
  • $$x = 2y + 1$$

  • Substitute this expression into equation (1):
  • $$3(2y + 1) + 4y = 24$$

  • Simplify:
  • $$6y + 3 + 4y = 24$$

    $$10y + 3 = 24$$

    $$10y = 21$$

    $$y = 2.1$$

  • Now, substitute $y$ back to find $x$:
  • $$x = 2(2.1) + 1 = 5.2$$

The solution is:

$$\text{Solution: } (5.2, 2.1)$$

3. Key Points to Remember
  • The substitution method is especially useful when one equation can be easily manipulated to express one variable in terms of the other.
  • Always check the solution by substituting the values back into both original equations to verify correctness.
  • This method can also be applied to nonlinear equations, but it may require more complex algebraic manipulation.
4. Advantages of the Substitution Method

The substitution method has several advantages:

  • It provides a clear and systematic approach to solving equations.
  • This method is particularly helpful when dealing with equations where one variable is already isolated or easily isolatable.
  • It can simplify problems that might be more complicated if tackled using other methods like elimination.
5. Practice Problems

To gain proficiency in the substitution method, try solving the following pairs of equations:

  • 1. $x + y = 10$ and $2x - y = 4$
  • 2. $5x + 3y = 15$ and $y = x + 2$
  • 3. $x - 3y = -6$ and $4x + y = 20$

By practicing these problems, you will enhance your understanding and application of the substitution method in solving linear equations.

Solve 2x + 3y = 11 and 2x – 4y = -24 and hence find the value of ‘m’ for which y = mx + 3.

Solution:

Given Equations:

1) $2x + 3y = 11$

2) $2x - 4y = -24$

Step 1: Solve for x and y using the method of substitution or elimination.

From equation (1): $2x + 3y = 11$

Rearranging for $x$: $2x = 11 - 3y$

$x = \frac{11 - 3y}{2}$

Substitute this value of $x$ into equation (2): $2x - 4y = -24$

Substituting $x = \frac{11 - 3y}{2}$ into the equation:

$2\left(\frac{11 - 3y}{2}\right) - 4y = -24$

Simplifying: $(11 - 3y) - 4y = -24$

$11 - 7y = -24$

$-7y = -24 - 11$

$-7y = -35$

$y = 5$

Step 2: Substitute $y = 5$ into equation (1) to find $x$.

Substitute $y = 5$ into $2x + 3y = 11$:

$2x + 3(5) = 11$

$2x + 15 = 11$

$2x = 11 - 15$

$2x = -4$

$x = -2$

Step 3: Find the value of $m$ for which $y = mx + 3$.

Now, we know that the values of $x = -2$ and $y = 5$. Substitute these into the equation $y = mx + 3$:

$5 = m(-2) + 3$

$5 = -2m + 3$

$-2m = 5 - 3$

$-2m = 2$

$m = -1$

Form the pair of linear equations for the following problems and find their solution by substitution method.

(i) The difference between two numbers is 26 and one number is three times the other. Find them.
(ii) The larger of two supplementary angles exceeds the smaller by 18 degrees. Find them.
(iii) The coach of a cricket team buys 7 bats and 6 balls for Rs. 3800. Later, she buys 3 bats and 5 balls for Rs. 1750. Find the cost of each bat and each ball.
(iv) The taxi charges in a city consist of a fixed charge together with the charge for the distance covered. For a distance of 10 km, the charge paid is Rs. 105 and for a journey of 15 km, the charge paid is Rs. 155. What are the fixed charges and the charge per km? How much does a person have to pay for travelling a distance of 25 km?
(v) A fraction becomes $\dfrac{9}{11}$, if 2 is added to both the numerator and the denominator. If, 3 is added to both the numerator and the denominator it becomes $\dfrac{5}{6}$. Find the fraction.
(vi) Five years hence, the age of Jacob will be three times that of his son. Five years ago, Jacob’s age was seven times that of his son. What are their present ages?

Solution:

(i) The difference between two numbers is 26 and one number is three times the other. Find them.
Let the two numbers be $x$ and $y$. The given conditions are: 1. The difference between the two numbers is 26, so $x - y = 26$. 2. One number is three times the other, so $x = 3y$. Substitute $x = 3y$ in the first equation: $3y - y = 26$ $2y = 26$ $y = 13$ Now substitute $y = 13$ in $x = 3y$: $x = 3 \times 13 = 39$ Thus, the two numbers are $39$ and $13$.
(ii) The larger of two supplementary angles exceeds the smaller by 18 degrees. Find them.
Let the two angles be $x$ and $y$. The given conditions are: 1. The angles are supplementary, so $x + y = 180$. 2. The larger angle exceeds the smaller by 18 degrees, so $x = y + 18$. Substitute $x = y + 18$ in the first equation: $(y + 18) + y = 180$ $2y + 18 = 180$ $2y = 162$ $y = 81$ Now substitute $y = 81$ in $x = y + 18$: $x = 81 + 18 = 99$ Thus, the two angles are $99^\circ$ and $81^\circ$.
(iii) The coach of a cricket team buys 7 bats and 6 balls for Rs. 3800. Later, she buys 3 bats and 5 balls for Rs. 1750. Find the cost of each bat and each ball.
Let the cost of each bat be $x$ and the cost of each ball be $y$. The given conditions are: 1. The cost of 7 bats and 6 balls is Rs. 3800, so $7x + 6y = 3800$. 2. The cost of 3 bats and 5 balls is Rs. 1750, so $3x + 5y = 1750$. Multiply the second equation by 2: $6x + 10y = 3500$ Now subtract the first equation from this: $(6x + 10y) - (7x + 6y) = 3500 - 3800$ $-x + 4y = -300$ $x = 4y + 300$ Substitute $x = 4y + 300$ in $7x + 6y = 3800$: $7(4y + 300) + 6y = 3800$ $28y + 2100 + 6y = 3800$ $34y = 1700$ $y = 50$ Now substitute $y = 50$ in $x = 4y + 300$: $x = 4 \times 50 + 300 = 200 + 300 = 500$ Thus, the cost of each bat is Rs. 500 and the cost of each ball is Rs. 50.
(iv) The taxi charges in a city consist of a fixed charge together with the charge for the distance covered. For a distance of 10 km, the charge paid is Rs. 105 and for a journey of 15 km, the charge paid is Rs. 155. What are the fixed charges and the charge per km? How much does a person have to pay for travelling a distance of 25 km?
Let the fixed charge be $x$ and the charge per km be $y$. The given conditions are: 1. For a distance of 10 km, the total charge is Rs. 105, so $x + 10y = 105$. 2. For a distance of 15 km, the total charge is Rs. 155, so $x + 15y = 155$. $(x + 15y) - (x + 10y) = 155 - 105$ $5y = 50$ $y = 10$ Now substitute $y = 10$ in $x + 10y = 105$: $x + 10 \times 10 = 105$ $x + 100 = 105$ $x = 5$ Thus, the fixed charge is Rs. 5 and the charge per km is Rs. 10. For a distance of 25 km, the total charge will be: $5 + 25 \times 10 = 5 + 250 = 255$ Thus, the person has to pay Rs. 255 for travelling a distance of 25 km.
(v) A fraction becomes $\dfrac{9}{11}$, if 2 is added to both the numerator and the denominator. If, 3 is added to both the numerator and the denominator it becomes $\dfrac{5}{6}$. Find the fraction.
Let the fraction be $\dfrac{x}{y}$. The given conditions are: 1. If 2 is added to both the numerator and the denominator, the fraction becomes $\dfrac{9}{11}$, so $\dfrac{x + 2}{y + 2} = \dfrac{9}{11}$. 2. If 3 is added to both the numerator and the denominator, the fraction becomes $\dfrac{5}{6}$, so $\dfrac{x + 3}{y + 3} = \dfrac{5}{6}$. From the first equation: $11(x + 2) = 9(y + 2)$ $11x + 22 = 9y + 18$ $11x - 9y = -4$ (Equation 1) From the second equation: $6(x + 3) = 5(y + 3)$ $6x + 18 = 5y + 15$ $6x - 5y = -3$ (Equation 2) Now, solve these two equations: Multiply Equation 1 by 5 and Equation 2 by 9: $55x - 45y = -20$ $54x - 45y = -27$ Subtract the second equation from the first: $55x - 54x = -20 + 27$ $x = 7$ Substitute $x = 7$ in Equation 2: $6(7) - 5y = -3$ $42 - 5y = -3$ $-5y = -45$ $y = 9$ Thus, the fraction is $\dfrac{7}{9}$.
(vi) Five years hence, the age of Jacob will be three times that of his son. Five years ago, Jacob’s age was seven times that of his son. What are their present ages?
Let Jacob’s present age be $x$ and his son’s present age be $y$. The given conditions are: 1. Five years hence, Jacob’s age will be three times his son’s age, so $x + 5 = 3(y + 5)$. 2. Five years ago, Jacob’s age was seven times his son’s age, so $x - 5 = 7(y - 5)$. Expanding both equations: From the first equation: $x + 5 = 3y + 15$ $x = 3y + 10$ (Equation 1) From the second equation: $x - 5 = 7y - 35$ $x = 7y - 30$ (Equation 2) Now, solve these two equations: Set Equation 1 equal to Equation 2: $3y + 10 = 7y - 30$ $10 + 30 = 7y - 3y$ $40 = 4y$ $y = 10$ Substitute $y = 10$ in Equation 1: $x = 3(10) + 10 = 30 + 10 = 40$ Thus, Jacob’s present age is 40 years and his son’s present age is 10 years.