[√(a+b)-√(a-b)]/[√(a+b)+√(a-b)]
=[√(a+b)-√(a-b)]^2/[a+b-(a-b)]
=[√(a+b)-√(a-b)]^2/(2b)
=[2a-2√(a^2-b^2)]/2b
=[a-√(a^2-b^2)]/