(2m+n)(m-3n)=2m²-3n²猜哪-5mn=0所以兆兆陵族戚2-12=5mn,mn=-2ab=(3m-2n)(m+4n)=3m²-8n²+10mn=3-32-20=-49|a|=√(3m-2n)²=√(9+16+24)=7,|b|=√(m+4n)²=√(1+64-16)=7cosθ=ab/(|a||b|)=-1夹角为180°.