49（x+y）²-9（x-y）²
=(7x+7y+3x-3y)(7x+7y-3x+3y)
=(10x+4y)(4x+10y)
=4(5x+2y)(2x+5y)
（x²-5）²+8（x²-5）+16
=(x²-5+4)²
=(x²-1)²
=(x-1)²(x+1)²