+491 What is the sum of the first 7 terms of the series −8+16−32+64−... ?
+128485 We have a geometric series.....the common ratio is 16/ - 8 = -2
The sum of the first n terms is given by:
first term [ 1 - common ratio^n ] / [ 1 - common ratio ]
So.....the sum is
-8 [ 1 - (-2)^(7) ] / [ 1 - (-2) ] =
-8 [ 1 - (-2)^7] / [ 3 ] =
-8 (129) / 3 =
-8 ( 43) =
-344
