(x+y)²-(9y)²=(x+y+3y)(x+y-3y)=(x=4y)(x-2y)(x+y)²-(9y)³=(x+y)²-(729y³)=(x+y)²-【√(729y³)】²=【x+y+√(729y³)】【x+y-√(729y³)】