Question

How do you write the direct variation equation if y=-9 when x= 3, find y when x=-5?

Answer 1

`y=(-3)x`
`y=15` when `x=-5`

Explanation:

If `y` varies directly as `x` then
`color(white)("XXX")y=c*x` for some constant `c`

We are told:
`color(white)("XXX")-9 = c*3`
`color(white)("XXX")rArr c=-3`
So the direct variation equation is `y=(-3)x`

and when #x=-5) #color(white)("XXX")y=(-3)*(-5) = 15#