Question

Find the value of x and y using cross multiplication method:
$-5x+3y=8$ and $4x+9y=5$

• A. (1, 1)
• B. (-1, 1)
• C. (1, -1)
• D. (-1, -1)

Answer 1

Answer: (B)

(-1, 1)

$-5x+3y=8$ ----- (1)

$4x+9y=5$ ----- (2)

First use formula for cross multiplication method:

$\frac{x}{b_1{c}_2-b_2{c}_1}=\frac{y}{c_1{a}_2-a_1{c}_2}=\frac{-1}{a_2{b}_2-a_2{b}_1}$

So, from equation (1) and (2) we can write the value of a, b and c.

$\frac{x}{3\times 5-9\times 8}=\frac{y}{8\times 4-(-5)\times 5}=\frac{-1}{-5\times 9-4\times 3}$

$\frac{x}{15-72}=\frac{y}{32+25}=\frac{-1}{-45-12}$

$\frac{x}{-57}=\frac{y}{57}=\frac{-1}{-57}$

$\frac{x}{-57}=\frac{-1}{-57}$

$x=\frac{-57}{57}=-1$

$\frac{y}{57}=\frac{-1}{-57}$

$y=\frac{-57}{-57}$

$y=1$

The solution is (-1, 1)