Mathematics
OpenStudy (anonymous):

I need help with these questions . please help me :/ What is the sum of an 8-term geometric series if the first term is 15 and the last term is -4,199,040? (3 points) Answer -3,599,175 -3,359,230 -3,119,285 -2,879,340 What is the sum of an 8-term geometric series if the first term is -11, the last term is 859,375, and the common ratio is -5? (3 points) Answer -143,231 -36,047 144,177 716,144 What is the 25th term of the arithmetic sequence 9, 15, 21, ? (3 points) Answer 151 152 153 154

OpenStudy (anonymous):

what is ur problem here?

OpenStudy (anonymous):

i mean, where r u stuck?

Parth (parthkohli):

Sum of a geometric series is: $$\Large \color{MidnightBlue}{\Rightarrow a {1 - r^n \over 1 - r} }$$ a is the first term. n is the number of terms r is the common ratio.

OpenStudy (anonymous):

Lets assume a common ratio r if the first term is 15 the nth term is 15 x r^(n-1) hence eighth term is 15 x r^7 Compute r from here. Use Parth's formula

Parth (parthkohli):

And.... $$\Large \color{MidnightBlue}{\Rightarrow a_n = a_1 + (n - 1)d }$$ To find the nth term of an arithmetic sequence.

OpenStudy (anonymous):

got it?

OpenStudy (anonymous):

question 3 is fairly easy. just find the common difference and use the arithmetic sequence formula to find the equation for finding the nth term. Then just plug in 25 in that equation.