微积分基本定理：d/dx ∫(a(x)→b(x)) ƒ(t) dt = b'(x)ƒ[b(x)] - a'(x)ƒ[a(x)]
导数乘法则：(uv)' = vu' + uv'
d/dx [x∫(0→x) ƒ(t) dt]
= x' * ∫(0→x) ƒ(t) dt + x * [∫(0→x) ƒ(t) dt]'
= ∫(0→x) ƒ(t) dt + x * [x' * ƒ(x) - 0' * ƒ(0)]
= ∫(0→x) ƒ(t) dt + xƒ(x)