Question

How many terms of the A.P.:-5, -1, 3, 7, ………..must be taken to give a sum of 400?​

Answer 1

Answer:

Given: A.P= -5, -1, 3, 7

SOL :

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

400 = n/2 {2(-5) + (n -1) 4}

400×2 = { -10 + 4n -4}

800 = n{ -14 +4n}

800 = n { -14+4n}

800 = -14n + 4n^2

4n^2 - 14n -800 = 0

2 ( 2n^2 - 7n -400) = 0

= ( 2n^2 - 32n+ 25n -400) =0

={2n (n-16) +25(n-16)}= 0

={ (n-16) +(2n+25)}=0

=n=16 , -25/2

The value of n cannot be negative

•Therefore, n=16

16no. of terms must be taken to give the sum of 400 in the given AP

PLZ MARK IT A BRAINLIEST.