Question

find the sum of the first 14 terms for a sequence starting with 2 ending with 120

Answer 1

Hi ,

If the sequence is in A.P ,

first term = a = 2 ,

last term = an = 120

number terms = n = 14

Sum of 14 terms = S14

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If a , l , n are first term , last term and

number terms in an A.P

Sum of n terms = Sn

Sn = n/2 [ a + an ]

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S14 = 14/2[ 2 + 120 ]

= 7 × 122

= 854

Therefore ,

sum of 14 terms = 854

I hope this helps you.

: )

Answer 2

Given, number of terms , n = 14
first term , a = 2
nth term = last term , Tn = 120

use formula ,
$\qquad\qquad S_n=\frac{n}{2}[a+T_n]$

= 14/2 [ 2 + 120 ]

= 7 [ 2 + 120 ]

= 7 × 122

= 854

hence sum of 14 terms = 854