Can someone check my work please?

Fine the sum of the arithmetic series -4 + 1 + 6 + 11 + ..... + 91.

I know that a1= -4, d=5, and a20=91.

S20= 20/2 (-4 + 91)
S20= 10 (87)
S20= 870
The sum of the arithmetic series is 870.

Your work is completely correct.

Yay!! Thank you so much for checking. Have a lovely day.

To check your work, we can use a formula to find the sum of an arithmetic series. The formula is:

Sn = (n/2)(2a1 + (n-1)d)

where Sn represents the sum of the series, n is the number of terms, a1 is the first term, and d is the common difference.

Let's plug in the values from your problem:

n = 20 (since you have 20 terms)
a1 = -4 (the first term you provided)
d = 5 (the common difference you provided)

Sn = (20/2)(2(-4) + (20-1)(5))
= 10(-8 + 19(5))
= 10(-8 + 95)
= 10(87)
= 870

Your calculation is correct! The sum of the arithmetic series -4 + 1 + 6 + 11 + ..... + 91 is indeed 870. Well done!