sum of a geometric sequence for a1=first term and n=which one and r is common ratio is
[tex]S_n= \frac{a_1(1-r^n)}{1-r} [/tex]
so
a1=1 students
r=4
n=5
[tex]S_n= \frac{1(1-4^5)}{1-4} [/tex]
[tex]S_n= \frac{(1-1024)}{-3} [/tex]
[tex]S_n= \frac{-1023}{-3} [/tex]
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