Question

The nth term of an AP (a ) is a = 8-5n, find the sum of first seven terms of the AP.
Rspl/3 mathematics Section-B

Answer 1

Step-by-step explanation:

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Answer 2

The sum of first seven terms of the AP series is -84

Step-by-step explanation:

• The nth term of an AP is  8-5 n

        $\mathbf{T_{n}=8 - 5 n}$

• So first, second, third and seventh term of series are find out by putting value of n are 1, 2 , 3 and 7.

        $\mathbf{T_{1}=8 - 5 \times 1= 3}$

        $\mathbf{T_{2}=8 - 5 \times 2= -2}$

        $\mathbf{T_{3}=8 - 5 \times 3= -7}$

        .

        .

        .

       $\mathbf{T_{7}=8 - 5 \times 7= -27}$

• From formula of sum of n term are given below

        $\mathbf{S_{n}=\frac{n}{2}(First\ term + Last\ term)}$

• Put in above equation

        n = 7

        First term = 3

        Last term = Seventh term = -27

• $\mathbf{S_{7}=\frac{7}{2}\times (3 -27)}$

       On solving, we will get

       $\mathbf{S_{7}=-84}$

       Sum of first seven term = -84