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Un =ar^n-1
b.
a = u1 = 10
r = u2/u1 = 20/10 = 2
U8 =ar^8 - 1
= 10 x 2^8-1
= 10 x 2^7
= 10 x 128
= 1280
U10 = ar^10 -1
= 10 x 2^9
= 10 x 512
= 5120
c.
Un = 10240
Un = ar^ n-1
10240 = 10 x 2^n-1
10240/10 = 2^n-1
1024 = 2^n-1
2^10 = 2^n-1
10 = n-1
n = 10 + 1
n = 11
2.
a = 8
U6 = 1/4
U6 = ar^6-1
1/4 = 8r^5
1/4/8 = r^5
1/32 = r^5
r = 1/32^1/5
r = 1/2