What is the sum of the first eight terms of the geometric series where a1 = 2 and r = −5?

Question

anthonyym · Answer

Geometric series is where you multiply by r each time. An example would be 1, 2, 4, 8, 16, where you multiply by 2 each time.
Now the first term is 2 and r=-5. You multiply by -5 each time.
a1 = 2
a2 = 2*-5 = -10
a3 = -10*-5 = 50
a4 = 50*-5 = -250
a5 = -250*-5 = 1250
a6 = -6250
a7 = 31250
a8 = -156250

anonymous · Answer

Thank you!

anthonyym · Answer

Now you add them together to find the sum. I do believe there's a formula for all this though. I've attached it

anonymous · Answer

Ya i have another formula but thank you!

anonymous · Answer

well i got 31,249

anthonyym · Answer

Well, I got -130208 and I checked with the formula and by adding the terms up.