Differentiation under the integral sign 积分符号内取微分
Differentiation under the integral sign is a useful operation in calculus. Formally it can be stated as follows:
This formula is the general form of the Leibniz integral rule and can be derived using the fundamental theorem of calculus. The (first) fundamental theorem of calculus is just a particular case of the above formula, for a(x) = a, a constant, b(x) = x and f(x, t) = f(t).