x∶y=5∶2 x=5y/2
M+N=2xy/(x²-y²)+(x²+y²)/(x²-y²)=(2xy+x²+y²)/(x²-y²)
=(x+y)²/(x-y)(x+y)
=(x+y)/(x-y)
=(5y/2+y)/(5y/2-y)
=(9y/2)/(3y/2)
=3=(9y/2)/(3y/2)這一步怎么來的?我是這么做的,不知道對(duì)不對(duì),用k表示一份數(shù)則5k+2k/5k-2k=7k/3k=7/3x∶y=5∶2 x=5y/2M+N=2xy/(x²-y²)+(x²+y²)/(x²-y²)=(2xy+x²+y²)/(x²-y²)=(x+y)²/(x-y)(x+y)=(x+y)/(x-y)=(5y/2+y)/(5y/2-y)=(7y/2)/(3y/2)=7/3 不用k表示一份數(shù)，直接把 x=5y/2帶進(jìn)去就可以了。