解4(a+b)^2-25(a-c)^2
=[2(a+b)]^2-[5(a-c)]^2
=[2(a+b)+5(a-c)][2(a+b)-5(a-c)]
=(2a+2b+5a-5c)(2a+2b-5a+5c)
=(7a+2b-5c)(2b+5c-3a)