View Compilation Error

0_0_30620208_25299.cpp: In function 'int main()':
0_0_30620208_25299.cpp:7:1: error: 'a' was not declared in this scope
 a = (x + y) / 2 ;
 ^
0_0_30620208_25299.cpp:8:1: error: 'b' was not declared in this scope
 b = (x - y ) / 2 ;
 ^