cmath头文件中常用函数有：
取绝对值，x需为double型，若为int型可乘一个1.01.double fabs(double x)向上取整，返回小于或等于x的最大整数，如floor(10.5) == 10.0 floor(-10.5) == -11.02.double floor(double x)向下取整，返回大于x的最小整数，如ceil(10.5) == 11.0 ceil(-10.5) == -10.03.double ceil(double x)计算幂值，返回r^p4.double pow(double r,double p)返回e^x次方5.double exp(double x)计算以e为底的对数，若要计算以10为底的对数：log10(x)若要计算以其他数为底的对数：log a(b) = log(b)/log(a)6.double log(double x)三角函数计算7.double sin(double x)cos(x) tan(x)反三角函数计算8.double asin(double x) acos(x) atan(x)四舍五入9.double round(double x)