What is the sum of an 8-term geometric series if the first term is -11 and the last term is 859,375, the common ratio being -5?

a) -143,231
b) -36,047
c) 144,177
d) 716,144

Question

What is the sum of an 8-term geometric series if the first term is -11 and the last term is 859,375, the common ratio being -5?

a) -143,231
b) -36,047
c) 144,177
d) 716,144

anonymous · Answer

@praxer Your answer was up for a moment and then disappeared for me =c

anonymous · Answer

do you have your math book handy? look in the example are of the section for "geometric series" equation

anonymous · Answer

*example area

praxer · Answer

follow the second formula... not the first one.. Okay ????? since r < 1 so the second one if the r > 1 than the first one..

here S is the sum and r is the common term and the mu is the first term and n is the no. of  terms.....

anonymous · Answer

@DemolisionWolf I take online courses and couldn't find the equations in my lessons, @praxer thank you very much for the help! Both times you've answered have helped me greatly ^^.

praxer · Answer

$$\huge \dfrac{-11*(1-(-5)^8)}{1+5}$$

the answer is 716144. Do check the calculations. Once again.......

anonymous · Answer

Why does the first -5 stay negative and the bottom -5 goes positive?

praxer · Answer

in the formula in the bottom it is 1-r since r is negative so 1-(-5)=1+5..... Got it ?..... :)

praxer · Answer

I skipped one step :P

anonymous · Answer

oh....I think i understand. Thank you!

praxer · Answer

$$\huge \frak WELCOME....... \ddot\smile $$