Let ( f ) be differentiable on ((a,b)) and continuous on ([a,b]), with ( f(a) = f(b) = 0 ). Suppose ( f(c) > 0 ) for some ( c ) in ((a,b)). Prove that there exist ( x_1 ) and ( x_2 ) in ((a,b)) such that ( f'(x_1) > 0 > f'(x_2) ).