Question 176 of 480
Find the value of x and y respectively if 3x - 5y + 5 = 0 and 4x - 7y + 8 = 0
- A. -5, -4
- B. -4,. -5
- C. 4, 5
- D. 5, 4
Correct Answer:
D
Explanation
To solve the system of equations given by:
1. \( 3x - 5y + 5 = 0 \) (Equation 1)
2. \( 4x - 7y + 8 = 0 \) (Equation 2)
we will use the method of substitution or elimination. Here, I will demonstrate the elimination method, which is often straightforward for systems of linear equations.
### Step 1: Rearranging the Equations
First, let's rearrange both equations to express them in the standard form \( Ax + By = C \).
From Equation 1:
\[
3x - 5y = -5 \quad \text{(Rearranging gives us this form)}
\]
From Equation 2:
\[
4x - 7y = -8 \quad \text{(Rearranging gives us this form)}
\]
### Step 2: Elimination of One Variable
Next, we will eliminate one of the variables. To do this, we can multiply the equations by suitable coefficients so that the coefficients of \( x \) or \( y \) become the same.
Let's eliminate \( x \). We can multiply Equation 1 by 4 and Equation 2 by 3:
- Multiply Equation 1 by 4:
\[
4(3x - 5y) = 4(-5) \implies 12x - 20y = -20 \quad \text{(Equation 3)}
\]
- Multiply Equation 2 by 3:
\[
3(4x - 7y) = 3(-8) \implies 12x - 21y = -24 \quad \text{(Equation 4)}
\]
### Step 3: Subtracting the Equations
Now, we can subtract Equation 3 from Equation 4 to eliminate \( x \):
\[
(12x - 21y) - (12x - 20y) = -24 - (-20)
\]
This simplifies to:
\[
-21y + 20y = -24 + 20
\]
\[
-y = -4
\]
### Step 4: Solving for \( y \)
Now, we can solve for \( y \):
\[
y = 4
\]
### Step 5: Substituting Back to Find \( x \)
Now that we have \( y \), we can substitute \( y = 4 \) back into one of the original equations to find \( x \). Let's use Equation 1:
\[
3x - 5(4) + 5 = 0
\]
\[
3x - 20 + 5 = 0
\]
\[
3x - 15 = 0
\]
\[
3x = 15
\]
\[
x = 5
\]
### Final Values
Thus, we have found:
- \( x = 5 \)
- \( y = 4 \)
### Conclusion
The correct option is **D. 5, 4**.
### Explanation of Other Options
- **Option A: -5, -4**: This option suggests both \( x \) and \( y \) are negative, which does not satisfy either of the original equations when substituted back.
- **Option B: -4, -5**: Similar to option A, substituting these values into the original equations will not satisfy them.
- **Option C: 4, 5**: This option has the values swapped and does not satisfy the equations when checked.
### Revision Summary
- Use the elimination method to solve systems of linear equations by making coefficients of one variable equal.
- Rearrange equations into standard form \( Ax + By = C \) for clarity.
- Substitute back to find the other variable after solving for one.
- Always check your solutions by substituting back into the original equations to ensure they hold true.