Find a1 if sn=-26,240 r=-3 n=8?

Presuming this is a geometric sequence and you have the sum of the first 8 terms and you know the common ratio and need to find the first term, then we use this equation:

Sn = a1(1 - r^n) / (1 - r)

We have Sn, r, and n, so substitute what we know and solve for a1 (which I'll change to just a to make things easier to read:

-26240 = a(1 - (-3)^8) / (1 - (-3))

-26240 = a(1 - 6561) / (1 + 3)

-26240 = a(-6560) / 4

-26240 = -1640a

a = 26240 / 1640

a = 16

The first term is 16.

Hope this helped. Please give best answer if it did.

s8 = a1(1-r^8)/(1-r)

-26240 = a1 (1-(-3)^8) /(1-(-3))

-26240 = a1 (1-(6561)) /4

-26240 = a1 (-6560/4)

-26240 = a1 * (-1640)

a1 = 26240/1640

a1 = 16

-26240 = a[1-(-3)^8]/4

a = 16

S8 = a r ^7________assuming a geometric progression

26420 = (-3)^7 a

a = - 26420 / 3^7

a = - 12 . 1

sn = a1*(r^n -1)/(r -1)

Substitute the given values and solve for a1.

-26240 = a1*((-3)^8 -1)/(-3 -1)

.. = a1*(6560/-4)

.. = -1640*a1

-26240/-1640 = a1 = 16

Geometric sum: Sn = a (r^n−1) / (r−1)

S₈ = −26240, r = −3

a ((−3)^8−1) / (−3−1) = −26240

a (6561−1) / −4 = −26240

a (6560)/−4 = −26240

−1640a = −26240

a = 16

Matthew, just read your comment to llaffer

The reason Google calculator gave you the wrong answer is probably because you entered it incorrectly.

(−3)^8 = 6561 ----> Correct

−3^8 = −(3^8) = −6561 ----> Incorrect

https://www.google.ca/search?source=hp&ei=mntlWszB...

Remember: any real number (whether positive or negative) raised to an even power will be positive.