Its Q 10 on page 139 Mixed Exercise 9D in Edexcel C2 Book:
10) The diagram shows part of the curve with equation y = 5 - 1/2x^2 for which y> (greater than or equal to) 0.
The point P(x,y) lies on the curve and O is the origin.
a) show that OP^2 = 1/4x^4 -4x^2 + 25