(i) (3+6+9+\cdots+a_{20})
A.P. with (a=3), (d=3), (n=20)
[ S_{20} = \dfrac{20}{2}\bigl[2\cdot3 + 19\cdot3\bigr] = 10(6+57) = 630 ]
Alternatively (a_{20}=3+19\cdot3=60), (S_{20}=\dfrac{20}{2}(3+60)=630)
Answer: (630)
11TH•Mathematics•Sequences And Series•Question 1
(i) \(3+6+9+\cdots+a_{20}\)
Sum the series:
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