Question

find the sum of 20 terms of an Ap 1,4,7,11....

Answer 1

a=1
d= a2-a1
=4-1
=3
n= 20
sn=?


Sn=n/2 (2a+(n-1)d)
s20=20/2 (2×1+(20-1)3)
s20=10 (2+54)
s20=560


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Answer 2

a = 1

cd = 3

Term 20 =n÷2 {2a +( n - 1 )D}

= 20÷2{ 2 × 1 + ( 20 - 1) × 3 }

= 10 { 2 + 19 × 3 }

= 10 {2 + 57}

= 10 × 59

= 590

Term 20 = 590