3(x²-y²)-1/2(xy-y)²=3x²-3y²-1/2(x²y²-2xy²+y²)=3x²-3y²-(½)x²y²+xy²-(½)y²=3x²-(7/2)y²-(½)x²y²...如果帮到你了。采纳是最好的感谢。