How would I expand this?

[A9876]

Homework Statement

r₁ = √(L²+(y-0.5d)²)
r₂ = √(L²+(y+0.5d)²)
Show that in the limit that both y and d are <<L r₂ - r₁ = yd/L

Homework Equations

The Attempt at a Solution

r₂ - r₁ =(√(L²+(y+0.5d)²)) - (√(L²+(y-0.5d)²))

Nevermind, I've figured it out but can't seem to be able to delete this post :s

 

[Simon Bridge]

I'd have tried substituting x=[(y-0.5d)/L]² and used the binomial approximation.
Oh you figured it out - rather than delete the post - how about posting your result: that way others who get stuck can benefit.