Question

find the sum of first 18 multiples of 3

Answer 1

First 18 multiples of 3 - 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54
And their sum is 513

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Answer 2

a = 3 d = 3
an = a + (n-1) d
3 + ( 18-1) 3
3+ 51
54
Sn = n/2( a + an)
9(3+51)
9*54
486