Question

1+4+7+10+...........+x=287find the value of x

Answer 1

Answer:

Step-by-step explanation:

Here a = 1, d = 3, an = x.

an= a + (n - 1)d

x = 1 + (n - 1)(3) = 3n - 2

Also, Sn = n/2(a+l)

287 = n/2(1+x)

287= n/2(1+3n-2)

574 = n(3n - 1)

3n^2 - n - 574=0

(3n + 41) (n - 14) = 0

n = 14,  -41/3

Rejecting n = -41/3

Therefore, x = 3n - 2 = 31*4 - 2 = 40.