An infinite geometric series has common ratio -1/2 and sum 45. What is the first term of the series?

Using the sum of an infinite series, we have :

a1 / [ 1 - r ] = the sum

Where a1 is the first term and r is the common difference.....so we have...

a1 / [ 1 - (-1/2) ] = 45 simplify

a1 / [ 3/2] = 45

a1 = 45 (3/2)

a1 = 67.5

Thank you, CPhill! Perfect!