(a) Prove that the solution to the linear integral equation $u(t)=a+\int_0^t k(s) u(s) d s$ solves the linear initial value problem $\frac{d u}{d t}=k(t) u(t), u(0)=a$.
(b) Use part (a) to solve the following integral equations
(i) $u(t)=2-\int_0^t u(s) d s$,
(ii) $u(t)=1+2 \int_1^t s u(s) d s$,
(iii) $u(t)=3+\int_0^t e^s u(s) d s$.