Question

Find the sum of the 1st 315 terms of an AP in which 2nd term is 2 and 7th term is 22

Answer 1

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Answer 2

Heya !!!


2 nd term = 2


A + D = 2 -------(1)


And,

7th term = 22


A + 6D = 22 --------(2)



From equation (1) we get,



A + D = 2


A = ( 2 - D ) -------(3)

Putting the value of A in equation (2)



A + 6D = 22



2 - D + 6D = 22



5D = 20


D = 20/5 = 4




Putting the value of D in equation (3)


A = 2 - D = 2 - 4 = -2



First term (A) = -2



And,


Common Difference ( D ) = 4



Sn = N/2 × [ 2A + ( N - 1 ) × D ]



S315 = 315 /2 × [ 2 × -2 + ( 315 - 1 ) × 4 ]




=> 315/2 × [ -4 + 1256 ]



=> 315/2 × 1252



=> 315 × 626



=> 197190



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