(x+y+1)(x-y+1)順番入れ替えます
=(x+1+y)(x+1-y)
x+1=Aとおくと
(A+y)(A-y)=A^2-y^2
A=x+1を戻す
(x+1)^2-y^2=x^2+2x+1-y^2
(4)x+2=Aと置き換えて下さい
あとはやり方同じです
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