①(2x+y-3z)²=(2x+y)²-6z(2x+y)+9z²=4x²+4xy+y²-12xz-6yz+9z²②y（x+y)+(x-y)²-x²-2y²=xy+y²+x²-2xy+y²-x²-2y²=-xy1、x+y-5=0 xy=6∴x+y=5x²+y²=(x+y)²-2xy=5²-12=132、由(1)得m²-2mn+n²=8 (3)由(2)得m²+2mn+n²=2(4)(3)+(4)得2m²+2n²=10∴m²+n²=5