9（a-b）^2-25(a+b)^2
=[3（a-b）+5(a+b)][3（a-b）-5(a+b)]
=[3a-3b+5a+5b][3a-3b-5a-5b]
=[8a+2b][-2a-8b]
=-4(4a+b)(a+4b)