[질문] 변형공식도 알려주세요

1.(x+y)²=(x－y)²+4xy2.(x－y)²=(x+y)²－4xy3.x²+y²=(x+y)²－2xy=(x－y)²+2xy4.x²+1/x²=(x+1/x)²-2=(x－1/x)²+2