提取公因式法7*(X+2Y)
7(x+2y)
7x+14y=7x+7y+7y=7(x+y)+7y=7(x+y+y)=7(x+2y)
7x+14y =7x+7y+7y =7(x+y)+7y =7(x+y+y) =7(x+2y)
7X+14Y=7(X+2Y)