Calculates value till integral limit
for ( int i = 1 ; i < interval_limit ; i ++ ) { if ( i % 3 == 0 ) sum = sum + 2 * func ( lower_limit + i * interval_size ) ; else sum = sum + 3 * func ( lower_limit + i * interval_size ) ; } return ( 3 * interval_size / 8 ) * sum ; }