Question

Find the sum : 1+(-2)+(-5)+(-8) + ..... + (-236)​

Answer 1

Answer:

-9400

Step-by-step explanation:

Given Series is :

1 + (-2) + (-5) + (-8) + .... + (-236)

Clearly, the series is in AP.

First Term = 1

Common difference d = (-5) - (-2) = -3

Last term = 236

Now,

It was asked to find the sum.

Sₙ = (n/2)[2a + (n - 1) * d

First We have to calculate n value.

aₙ = a + (n - 1) * d

⇒ -236 = 1 + (n - 1) * (-3)

⇒ -236 = 1 - 3n + 3

⇒ -236 = 4 - 3n

⇒ -240 = -3n

⇒ n = 80

Thus,

S₈₀ = (80/2)[2(1) + (80 - 1) * (-3)]

    = 40[2 + 79 * -3]

   = 40[-235]

  = -9400

Therefore,

Sum = -9400

Hope it helps!

Answer 2

Answer: -9400

Step-by-step explanation:

Refer the attachment for better understanding