1)(1/x-1/y):(1/x+1/y)=(y-x)/xy:(y+x)/xy=(y-x)/xy*xy/(y+x)=(y-x)/(y+x)
2)(1/x+1/y):(1/x-1/y)=(y+x)/xy:(y-x)/xy=(y+x)/xy*xy/(y-x)=(y+x)/(y-x)
3)(y-x)/(y+x)-(y+x)/(y-x)=(y²-2xy+y²-x²-2xy-y²)/(y²-x²)=-4xy/(y²-x²)
4)[4xy/(y²-x²)]^-1=(y²-x²)/4xy
5)-4xy/9y²-x²)*(y²-x²)/4xy=-1