View Compilation Error

1_549_361_18651.cpp: In function `double gao()':
1_549_361_18651.cpp:65: error: `x1' undeclared (first use this function)
1_549_361_18651.cpp:65: error: (Each undeclared identifier is reported only once for each function it appears in.)
1_549_361_18651.cpp:65: error: `y1' undeclared (first use this function)
1_549_361_18651.cpp:65: error: `x2' undeclared (first use this function)
1_549_361_18651.cpp:65: error: `y2' undeclared (first use this function)
1_549_361_18651.cpp:66: error: `x3' undeclared (first use this function)
1_549_361_18651.cpp:66: error: `y3' undeclared (first use this function)