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

Question

answers:
870
779
95
87

Answer 1

this is an arithmetic progression.
a = -4
d = 5
-4 + (n-1) * 5 = 91
=> n=20
=> sum is 10 * 87 = 870

Answer 2

-4+1+6+11+...+91

= -4 + (1+6+11+...+91)

= -4 + (19/2)(2 + 18*5)

= -4 + (19/2) (92)

= -4 + 46*19

= -4 + 874

= 870

So, 870 is the correct answer to the given problem.

Answer 3

first term ,a= -4
common diff. , d=+5
n'th term =91
a+(n-1)d=91
-4+(n-1)5=91
(n-1)5=95
n=19+1=20

sum of the arithmetic series= n/2(2a +(n-1)d)
=20/2 (-8 + 19(5))
=10(87)=870