Выражение (y/x-y + x/x+y) : (1/x^2 + 1/ y^2) - y^4/ x^2-y^2

(y/x-y + x/x+y) : (1/x^2 + 1/ y^2) - y^4/ x^2-y^2 = (y(x+y)+x(x-y)/(x-y)(x+y))/(x^2+y^2/x^2y^2)-y^4/(x-y)(x+y) = (xy+y^2+x^2-xy/(x-y)(x+y))/(x^2+y^2/x^2y^2)-y^4(x-y)(x+y) = (x^2+y^2/(x-y)(x+y))/(x^2+y^2/x^2y^2)-y^4(x-y)(x+y) = (x^2+y^2)(x^2y^2)/(x^2+y^2)(x-y)(x+y)-y^4/(x-y)(x+y) = x^2y^2/(x-y)(x+y)-y^4/(x-y)(x+y) = x^2y^2-y^4/(x-y)(x+y) = y^2(x^2-y^2)/(x-y)(x+y) = y^2(x-y)(x+y)/(x-y)(x+y) = y^2    

((х+у+х-у)/(x^2-y^2))/(x/(  x^2-y^2)) = (2x/(x^2-y^2))*  ((x^2-y^2)/x) = x

-0,7*1000+5*100+70=-700+500+70=-130

y`+ 4y+4-2(y`+2y+1)= y`+4y+4-2y`-4y-2=-y`+2.   y`-это игрек в квадрате

второе и третье не пойму: зачем два знака равно?

6(а`-3a-2a+6)-4(a`-6a+9)=6a`-18a-12a+36-4a`+24a-36=2a`-6a=2a(a-3)

a`-2a+1+2a-2+1=a`

1-2a+a`+2a-2a`+a`=1

x`+2x+1-4x-4+4=x`-2x+1=(x-1)`

x`+2xy+y`-2x`+2y`+x`-2xy+y`= 4y`