View Compilation Error

0_0_12747497_32234.cpp: In function `double merge(int, int)':
0_0_12747497_32234.cpp:58: error: `fabs' undeclared (first use this function)
0_0_12747497_32234.cpp:58: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_12747497_32234.cpp: In function `int main()':
0_0_12747497_32234.cpp:90: error: `sqrt' undeclared (first use this function)