" 13."xy(z^(2)+1)+z(x^(2)+y^(2))

If xy + yz + zx = 1 , show that x/(1-x^(2)) +y/(1-y^(2)) + z/(1-z^(2))= 4xyz/((1-x^(2))(1-y^(2)) (1-z^(2)))

Prove that |(x^(2),x^(2)-(y-z)^(2),yz),(y^(2),y^(2)-(z-x)^(2),zx),(z^(2),z^(2)-(x-y)^(2),xy)|=(x-y)(y-z)(z-x)(x+y+z)(x^(2) + y^(2) + z^(2)) .

xy,xz,x^(2)+1y^(2)+!,yz,xyyz,z^(2)+1,xz]|=1+x^(2)+y^(2)+z^(2)

Simplify: (x+y-2z)^(2)-x^(2)-y^(2)-3z^(2)+4xy