a) You area told that y is proportional to x2 and that when y = 75, x = 5. Find a formula for y in terms of x. y = x b) Find the value of y when x = 3. c) Find the value of x when y = 1200.

a) y is proportional to x^2, when x is 5, y is 75

Therefore: 75 = (something multiplied by) 5^2

75 = 3 x 25

Hence y = 3x^2

b) y = 3 multiplied by 3^2 

= 27

c) 1200 = 3x^2

1200/3 = x^2

400 = x^2

Undo the square of x (on both sides!)

x = 20