Question

7
th
term of an AP is 40. The sum of its first 13
th
terms is​

Answer 1

Answer:

Sum of 13 terms is 520.

Step-by-step explanation:

Given:

Seventh term of an AP = 40

To find: Sum of first 13 terms.

Let, a be the first term and d be the common difference.

As we know that,

aₙ = a + (n-1)d

a₇ = 40

a + ( 7 - 1 )d = 40

a + 6d = 40  ...(i)

Also,

Sₙ =  n/2(2a + (n - 1)d)

S₁₃ = 13/2(2a + (13 - 1 )d)

     = 13/2(2a + 12d)

    = 13/2 × 2 (a + 6d)

    = 13 × 40       ( From equation (i) )

    = 520

∴  Sum of 13 terms is 520.

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