Question

Find sum of first 20 terms of an AP whoes nth term is 4n-1

Answer 1

Answer:

Step-by-step explanation:

Tn = 4n-1

-----------------

If n = 1 ;

T1 = 4(1)-1 = 3

a = 3

----------------

If n = 2 ;

T2 = 4(2)-1 = 7

a+d = 7

---------------

3 = 7-d

d = 4

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Now, in a A.P

a = 3

d = 4

n = 20

S20 = ?

Sn = (n/2)[2a+(n-1)d]

S20 = (20/2) [(2×3)+(20-1)4]

= 10 ×(6+76)

= 10×82

= 820

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