Question

If the first term of an A.P.is 5, its common difference is - 4 and its last term is -35,then find its sum

Answer 1

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

n=35, D=4, a=5

Sn=35/2 (2*5+(35-1) 4)

35/2 (10+136)

35*146/2

35*73

=answer

Answer 2

Answer:

n = 17/2 or 8.5 ; Sn = -340

Step-by-step explanation:

Given :-

a = 5

d = -4

last term (an) = -35

To find :-

Sn = ?

Proof :-

an = a + (n - 1) × d

-35 = 5 + (n - 1) × (-4)

-35+5 = -4n + 4

-30 = -4n + 4

-30 - 4 = -4n

-34 = -4n

n = 34/4

»»»n = 17/2 or 8.5

Sn = n/2 [2a + (n - 1) × d]

= (17/2)/2 [2×5 + (8.5 - 1) × (-4)]

= 17 [10 + 7.5 × (-4)]

= 17 × (-20)

»»»Sn = -340

I hope it helps.....