Math, asked by keerthanadileep02, 1 year ago

find 3 + 6 + 9 +...... + 420​

Answers

Answered by khushichiya
2

an=a+(n-1)d

420=3+(n-1)3

417=(n-1)3

(n-1)=139

n=140

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

=70(6+417)

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