Question

find the 20th term of the arithmetic progression whose third term is 7 and 8th term is 17?

Answer 1

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Answer 2

Answer:

Step-by-step explanation:

=>Sn = a + (n-1)d

where

Sn = nth term

a = first term

d = difference

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Given

======

=> 3rd term = 7

=> a +(3-1)d = 7

=> a + 2d = 7____(1)

Also

=> 8th term = 17

=> a + (8-1)d = 17

=> a + 7d = 17_____(2)

Subtracting equation (1) from (2)

============================

=>  a+2d=7 ________(1)

     a+7d=17________(2)

    (-) (-)   (-)

______________

=> -5d = -15

=> d = -15/-5 = 3

Putting the value of d in equation (1)

===============================

=> a+ 2(3) = 7

=> a = 7-6 = 1

__________________________

=> 20th term of AP = a +(20-1)d

= -1 + 19×3 = -1 +57 = 56

-_-_-_-_-_-_--_-_-_-_-_-_-_-_-_-

=> nth term = a + (n-1)d

________________________

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