Question

The variable z varies jointly with x and y. also, z = -75 when x = 3 and y = -5. what equation describes this variation?

Answer 1

The equation describing variation will be found as follows:
zαxy
z=kxy
where k is the proportionality constant
k=z/xy
when x=3, y=-5 then z=-75
k=-75/(-5*3)=5
hence the formula will be:
z=5xy