`21)(1/3x+1/2y)^2`

`=(1/3x)^2+2*1/3x*1/2y+(1/2y)^2`

`=1/9x^2+1/3xy+1/4y^2`

`22)(2x-1/y)^2`

`=(2x)^2-2*2x*1/y+(1/y)^2`

`=4x^2-4x/y+1/y^2`

`23)(5x-4/y)^2`

`=(5x)^2-2*5x*4/y+(4/y)^2`

`=25x^2-40x/y+16/y^2`

`24)(3x-2/(3y))^2`

`=(3x)^2-2*3x*2/(3y)+(2/(3y))^2`

`=9x^2-4x/y+4/(9y^2)`

`25)(2/(3x)-y)^2`

`=(2/(3x))^2-2*2/(3x)*y+y^2`

`=4/(9x^2)-4/3*y/x+y^2`

`26)(2/x^2+x)^2`

`=(2/x^2)^2-2*2/x^2*x+x^2`

`=4/x^4-4/x+x^2`

`27)(0,5-1/(xy))^2`

`=(0,5)^2-2*0,5*1/(xy)+(1/(xy))^2`

`=0,25-1/(xy)+1/(x^2y^2)`

`28)(7x-1/x)^2`

`=(7x)^2-2*7x*1/x+(1/x)^2`

`=49x^2-14+1/x^2`

`29)(2x+y-3z)^2`

`=[(2x+y)-3z]^2`

`=(2x+y)^2-2*(2x+y)*3z+(3z)^2`

`=4x^2+4xy+y^2-12xz-6yz+9y^2`

`30)(1/2xy-x+y)^2`

`=[1/2xy-(x-y)]^2`

`=(1/2xy)^2-2*1/2xy(x-y)+(x-y)^2`

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

`=1/4x^2y^2-x^2y+xy^2+x^2-2xy+y^2`