Now comput B * r for evaluating expression A * ( B * r ) - ( C * r )
let br = new Array ( N ) ; br . fill ( 0 ) ; for ( let i = 0 ; i < N ; i ++ ) for ( let j = 0 ; j < N ; j ++ ) br [ i ] = br [ i ] + b [ i ] [ j ] * r [ j ] ;