已知两个函数 ，求它们在指定区间的定积分，要求用矩形法编制求解定积分的通用函数（函数中可用函数型参数或函数指针型参数）。-The known two function, let them in the designated the definite integral interval, the requirement with rectangle method of the definite integral solution of general function (function type parameters by functions or function pointer type parameters).