分解因式：a^4-16 = (a^2+4)(a^2-4) = (a^2+4)(a+2)(a-2) 4(a+b)^2-9(a-b)^2 = [2(a+b)+3(a-b)]·[2(a+b)-3(a-b)] = (5a-b)(5b-a)
