Differentiation

you can check by putting in values for a,b, & x to see if comes to b²

but i have to show that when it expands it comes to b^2

it is to check if something has gone wrong with your differentiating...*

Given that x^2 / ax+b, (a, b doesn't equal zero) show that d^2y/dx^2 = 2b^2 /(ax+b)^3

i get to this

d^2y/dx^2 = {(2ax + 2b)(ax + b) - (ax^2 + 2bx)(2a^2x + 2ab)}/(ax+b)^3

when i expand this it's nowhere near just b^2

would anybody care to tell me what i'm doing wrong

this what i get if i expand the numerator

2b^2 + 2a^2x + 4abx - 4a^2bx^2 - 2a^3x^3 -2ab^2x