請問一題六月數學JJ

(x+y)^2=6xy

(x-y)^2=2xy

(x-y)/(x+y)= +根号1/3或者-根号1/3

請問此題答案如何推演??   

开平方根答案应该有正负2个的 = =..

[(x-y)/(x+y)]^2=2xy/6xy=1/3        (x-y)/(x+y)=+-根号3/3