Question

Variable y varies directly with x2, and y = 75 when x = 5.

What is the value of y when x = 3?

y =

Answer 1

$y=kx^2 \\ \\ y=75 \hbox{ when } x=5 \\ 75=k \times 5^2 \\ 75=k \times 25 \\ k=\frac{75}{25} \\ k=3 \\ \\ y=3x^2 \\ \\ \hbox{when } x=3: \\ y=3 \times 3^2=3 \times 9=27 \\ \boxed{y=27}$