1、cosθ=4/5,sinθ=3/5,|a×b|=|a||b|sinθ=3
[(a+b)×(a-b)]^2=[2(a×b)]²=4|a×b|²=36
[(a-3b)×(b-3a)]^2=[-8(a×b)]²=64|a×b|²=576