Can u plz help me with this precalc problem?

Question

y varies jointly with the cube of x and the square of z and inversely with the square of the sum of x and z if y = 72 when x= 2 and z=3 , then find the value of y when x= -1 and z=5.....thx...... I am very lost plz explain all work and answers in detail thx

Note jointly means multiplied inversely in devide

Answer 1

Just set up the equation. Any time you're dealing with variation instead of equality, there will be a constant of proportionality. Let's call it k. So, your problem says:

y = kx^3z^2/(x + z)^2

You're told that

72 = k(2^3)(3^2)/(2 + 3)^2 = 72k/25

k = 25

Now that you know k, you have the whole equation. You can find y just by plugging in.