18o
?easy
,axaxffxbbyyxy=fx
out
s?,x?f?(?y?)?dy?dz?+?y?f?(?x?)?dz?dx?-?z?[?b+?f?(?x?+?y?)?]?dx?dy?=?o,f?(?x?)?(-???,+???)?,f?(?1)?=?a(?a,b?)?,f?(?2o1o)
:
p=xf(y)q=yf(x)r=-z[b+f(x+y)]?pxqyrzo?f(y)+f(x)-b-f(x+y)=o?f(x+y)=f(x)+f(y)-b?f(2)=2f(1)-b=2a-b,f(3)=f(2)+f(1)-b=3a-2b?f(4)==4a-3b??f(n)=na-(n-1)bf(2o1o)=2o1oa-2oo9b