【例6.13】梯形法求积分的函数integer()是通用函数，可求任一函数的定积分。不同的函数有不同的解析式，

//该解析式决定了自变量在每一个小积分区间端点处的函数值。函数 integer()以一个指向函数的指针为参数，

//由该参数调用欲求定积分的函数，另两个参数是积分上下限。-[Example 6.13] the trapezoidal method to seek the integral function integer () is a universal function, rectifiable any definite integral of a function. Different functions have different analytical formula

//Parse type determines the value of the function of independent variables in each small integral interval endpoints. Function integer () to a pointer to a function as a parameter,

//Desire definite integral function is called by the parameters, the other two parameters are the limits of integration.