令x=an+1 y=1
f(an+1)+f(1)=f(an)
f(an+1)=f(an)-f(1)
而f(a(n+1))=f(an+1)
则 f(a(n+1)=f(an)-f(1)
f(an)=f(a(n-1))-f(1) ..........(1)
f(a(n-1))=f(a(n-2))-f(1)..........(2)
......
f(a2)=f(a1)-f(1) ............(n-1)
将上述(1)、(2)、...(n-1)式相加,得
f(an)=f(a1)+(n-1)f(1)