What is the recursive formula for this geometric sequence? 4, -12, 36, -108, ...
2 answers:
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Answer:
Answer: Sum of first 90 terms is Option d. 11655
Step-by-step explanation:
We have to find the sum of first 90 terms of the sequence -4,-1,2,5,8......
First we find the 90th term of the sequence
As we know the formula A(n)= A(1)+(n-1)d
Given from the sequence
A(1) = (-4)
n=90
d = 3
So A(n) = -4+(90-1)3
= (-4)+89×3
= 263
Now we know formula for the sum of any sequence is
Sum S(n) =
Now we know n= 90
A(1) = -4
A(n) =263
Now we put the value in the formula
S(n)=
= 45×259
= 11655