Question

if the sum of the first four terms of an AP is 40 and that of the first 14 terms is 280. find the sum of n terms with an explanation

Answer 1

Answer with Solution -

Sn = sum of n terms of an A.P. = n/2 [ 2a + (n-1)d]

A/Q

S₄ = 40 = 4/2 [2a + 3d] = 2a + 3d = 20 ...........(1)

and

S₁₄ = 280 = 14/2 [2a + 13d] = 2a + 13d = 40 .........(2)

solving (1) and (2)

gives a = 7 and d= 2

so Sn = n/2[ 2(7) + (n-1)2] = 7n + n² - n = n² + 6n

hope it helps.

Please mark my answer brainliest.

Answer 2

Step-by-step explanation:

hope it will help you thank you