Question

求等比数列一分之九,二分之九,四分之九,八分之久,...的前10项和.

Answer 1

a2-a1=a3-a2

-(x+1)^2+4(x+1)-2 = (x-1)^2-4(x-1)+2

2x^2+2-8x+4=0

x^2-4x+3=0

x=1 or 3

if x=1

a1=f(2)=4-8+2=-2

a3=f(0) =2

an = -2+(n-1)2

= 2n-4

Sn = n(a1+an)/2 = n(n-3)

if x=3

a1=f(4)=16-16+2=2

a3=f(2)=4-8+2=-2

an =2-2(n-1)

=4-2n

Sn= (3-n)n