动量定理:m1v1+m2v2=m1v3+m2v4; 动能定理:(1/2)m1v1v1+(1/2)m2v2v2=(1/2)m1v3v3+(1/2)m2v4v4 v3 v4怎么解啊???有结果的也可以告诉下,实在是难解。~