Hello,
If you don't know how to calculate the sum of a geometric progression,
the solution is the file joined.
else
[tex]ratio= \frac{-1}{3} \\
u_{1} =5\\
u_{2} =5* \frac{-1}{3} \\
u_{3} =5* (\frac{-1}{3})^2 \\
u_{4} =5* (\frac{-1}{3})^3 \\
u_{n} =5* (\frac{-1}{3})^{n-1} \\
sum= u_{1} * \dfrac{1}{1-( \frac{-1}{3})} =5*3/4=15/4=3.75
[/tex]
