View Compilation Error

0_0_35577593_4071.cpp: In function 'int main()':
0_0_35577593_4071.cpp:26:27: error: 'sqrt' was not declared in this scope
                 l1=sqrt(l1);
                           ^
0_0_35577593_4071.cpp:33:42: error: 'sqrt' was not declared in this scope
             double tt1=sqrt(lx*lx+300*300)/v1+2*t1+lx/v1;
                                          ^
0_0_35577593_4071.cpp:45:27: error: 'sqrt' was not declared in this scope
                 l1=sqrt(l1);
                           ^
0_0_35577593_4071.cpp:51:42: error: 'sqrt' was not declared in this scope
             double tt1=sqrt(lx*lx+300*300)/v1+sqrt((300-lx)*(300-lx)+300*300)/v1+3*t1;
                                          ^