○S=1+42+x+10x3+
-125=
・・・・+(3η-2)-1
x + 4x² + 17x² + "...+ (3h-5) x² + + (3n-2) x²-
S(1-x)=1+(3x+3x²+3×3+…+3でリ(34-2)で
⇒初項3x,公比x,填数n-1
nt
3x(x-1)
= (3n-2) x" +1
n
x-1
32"-32. — (31-2) x²rt
(35-2)x²+)
(22-2)
a-1
(3n-2) x+
=
11
x-x
3x(x-1)-(3m-2)(x-1)x+(x-1)
x-1
3x²-3x-(3-2)(x+_x)+x-1
3x²-2x-1
x-1