9(m+n)²-16(m-n)²
=[3(m+n)]²-[4(m-n)]²
=[3(m+n)+4(m-n)][3(m+n)-4(m-n)]
=(3m+3n+4m-4n)(3m+3n-4m+4n)
=(7m-n)(7n-m)
这里主要考察平方差公式：a²-b²=(a+b)(a-b)