9(a+b)²-16(2a-b)²
=[3(a+b)]²-[4(2a-b)]²
=(3a+3b+8a-4b)(3a+3b-8a+4b)
=(11a-b)(7b-5a)
(9k²+1)²-36k²
=(9k²+1)²-(6k)²
=(9k²+1+6k)(9k²+1-6k)
=(3k+1)²(3k-1)²
(3) 由余弦定理：a²=b²+c²-2bccosa
a²-b²-c²-2bc=-2bccosa-2bc
=-2bc(cosa+1)
因为 cosa>=-11 所以cosa+1>=0
所以-2bc(cosa+1)