Answer:
Sum of the first five terms of the geometric sequence in which a1=5 and r=1/5 is [tex]\mathbf{S_n=\frac{781}{125} }[/tex]
Step-by-step explanation:
We need to find sum of the first five terms of the geometric sequence in which a1=5 and r=1/5
The formula used to find sum of the geometric sequence is: [tex]S_n=\frac{a(r^n-1)}{r-1}[/tex]
Where a is the first term, r is the common ratio and n is the number of terms
Now finding sum of the first five terms of the geometric sequence
We have a=5, r=1/5 and n=5
Putting values in the formula:
[tex]S_n=\frac{a(r^n-1)}{r-1}\\S_n=\frac{5((\frac{1}{5}) ^5-1)}{\frac{1}{5}-1}\\S_n=\frac{5(\frac{1}{3125}-1)}{\frac{1-5}{5}}\\S_n=\frac{5(\frac{1-3125}{3125})}{\frac{-4}{5}}\\S_n=\frac{5(\frac{-3124}{3125})}{\frac{-4}{5}}\\S_n=5(\frac{-3124}{3125})}\times{\frac{-5}{4}}\\S_n=\frac{781}{125}[/tex]
So, sum of the first five terms of the geometric sequence in which a1=5 and r=1/5 is [tex]\mathbf{S_n=\frac{781}{125} }[/tex]
