Question

Find Sn for the given arithmetic series.
a1= -100, d = –2, n = 21

Answer 1

Answer:

$-2520$

Step-by-step explanation:

For arithmetic series, $S_n$ means that sum of first n terms.

The formula is:

$S_n=\frac{n}{2}[2a_1+(n-1)d]$

Where

$a_1$  is the first term (given as -100)

n is the number of terms (given as 21)

d is the common difference (given as -2)

So, we can plug in the values we know and find the sum of first 21 terms:

$S_n=\frac{n}{2}[2a_1+(n-1)d]\\S_{21}=\frac{21}{2}[2(-100)+(21-1)(-2)]\\S_{21}=10.5[-200+20(-2)]\\S_{21}=10.5[-240]\\S_{21}=-2520$

So the sum is -2520