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We have to use the formula [tex]n = log_{r} (1+\frac{S_{n}(1-r) }{a})[/tex] to find the number of terms of a finite geometric sequence.
If a be the first term of a finite sequence, r be the common ratio between consecutive terms and n be the number of terms.
So, we have to use the formula of sum of sequence and then calculate it to reduce the equation to find the value of number of terms, that is n.
Then, Sum of the sequence (Sn) = [tex]\frac{a(1-r^{n}) }{1-r}[/tex]
Here, in the given problem,
Sum(Sn) = 280, First term of the sequence(a) = 40, Common ratio(r) = 0.75
So, Sn = [tex]\frac{a(1-r^{n}) }{1-r}[/tex]
⇒ [tex]1-r^{n} =\frac{S_{n}(1-r) }{a}[/tex]
⇒ [tex]r^{n} =1+\frac{S_{n}(1-r) }{a}[/tex]
⇒ [tex]n = log_{r} (1+\frac{S_{n}(1-r) }{a})[/tex]
Now you have to put the values and get the number of terms.
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