（1）25x²-16y²
=(5x)²-(4y)²
=(5x+4y)(5x-4y)
（2）(a-b)(x-y)-(b-a)(x+y)
=(a-b)(x-y)+(a-b)(x+y)
=(a-b)(x-y+x+y)
=2x(a-b)
（3）x³-9x
=x(x²-9)
=x(x+3)(x-3)
（4）16x²-1
=(4x)²-1
=(4x+1)(4x-1)
（6）4y²-9(x+y)²
=(2y)²-[3(x+y)]²
=(2y+3x+3y)(2y-3x-3y)
=(3x+5y)(-3x-y)
=-(3x+5y)(3x+y)