If a n = 1 0 1 n , what is the value of the summation below?
n = 1 ∑ 1 0 1 1 − 3 a n + 3 a n 2 a n 3
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I can't believe I used a whole day to solve this instead of solving elegantly like you TT
Same technique here.
Almost same
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First notice, a 1 0 1 − n = 1 0 1 1 0 1 − n = 1 − 1 0 1 n = 1 − a n
⇒ a n = 1 − a 1 0 1 − n
1 − 3 a n + 3 a n 2 a n 3 + 1 − 3 a 1 0 1 − n + 3 a 1 0 1 − n 2 a 1 0 1 − n 3
= ( 1 − a n ) 3 + a n 3 a n 3 + ( 1 − a 1 0 1 − n ) 3 + a 1 0 1 − n 3 a 1 0 1 − n 3
= ( 1 − a n ) 3 + a n 3 a n 3 + a n 3 + ( 1 − a n ) 3 ( 1 − a n ) 3
= 1
Here, the summation of n th and ( 1 0 1 − n ) th terms are 1 . Thus the sum from n = 1 to 1 0 0 is 5 0 .
The 1 0 1 th term is 1 − 3 a 1 0 1 + 3 a 1 0 1 2 a 1 0 1 = 1 − 3 + 3 1 = 1 .
So, total sum = 5 1