A student’s derivation of the formula for the sum of a finite geometric series is shown.

Question

Step 1: 

Step 2: 

Step 3:  .......................................

Step 4: 

Step 5: 

A) a(ar^n-1)

B) a-ar^n

C) a(1-ar^n)

D) ar^n-a

Answer 1

B) a -ar^n

after subtracting, every term cancels except the 1st of Sn and last of rSn