16（x-y）（x-y）-25（x+y）（x+y)
=[4(x－y)]²－[5(x＋y)]²
=(4x－4y＋5x＋5y)(4x－4y－5x－5y)
=(9x＋y)(﹣x－9y)
=﹣(9x＋y)(x＋9y)
x的4次方+4乘y的4次方
=x^4＋4x²y²＋4y^4－4x²y²
=(x²＋2y²)²－(2xy)²
=(x²＋2xy＋2y²)(x²－2xy＋2y²)