./test.cpp: In function 'int main()':
./test.cpp:6:24: error: expression cannot be used as a function
printf("%.4f\n"(a+c)*b/2);
^
./test.cpp:5:34: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
scanf("%lf %lf %lf",&a,&b,&c);
^