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问题: 又是因式分解

x^n+1+x^n
x^n+1+x^n-1
x^my^m+3-(x-y)^2x^my^m+1(這裡x^m乘以y^m+1)
求方程x^2-y^2=15的正整數解

解答:

x^(n+1)+x^n =(x^n)*(x+1)
x^(n+1)+x^(n-1)=[x^(n-1)]*(x^2+1)
x^my^m+3-(x-y)^2x^my^m+1(這裡x^m乘以y^m+1)
=(x^my^m)[y^3-(x-y)^2*y]
=(x^my^m)[xy(2y-x)]
=xy(2y-x)(x^my^m)

求方程x^2-y^2=15的正整數解
x^2-y^2=15→
(x-y)(x+y)=3×5
x-y=3,x+y=5
∴x=4,y=1