已知方程组{(X+Y)-3(X-Y)=7,2(X+Y)+(X-Y)=-7}则(X+Y)(X-Y)=
问题描述:
已知方程组{(X+Y)-3(X-Y)=7,2(X+Y)+(X-Y)=-7}则(X+Y)(X-Y)=
答
设x+y=m,x-y=n.则:已知的方程组变为:m-3n=7,2m+n=0,解这个新的方程组得:m=1,n=-2所以:x²-y²=(x+y)(x-y)=mn=-2