f'(x) = lim [ (x+h-x) / h.(√(x+h) + √x) ] f'(x) = lim [ 1 / (√(x+h) + √x) ] what do I do with ℎ → 0 ? f'(x) = 1 / (√(x+0) + √x) f'(x) = 1 / (2√x)