Now comput A * ( B * r ) for evaluating expression A * ( B * r ) - ( C * r )
int axbr [ N ] = { 0 } ; for ( int i = 0 ; i < N ; i ++ ) for ( int j = 0 ; j < N ; j ++ ) axbr [ i ] = axbr [ i ] + a [ i ] [ j ] * br [ j ] ;