Question

What is the sum of the terms in the sequence
3, 6, 9, …, 99?
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Answer 1

`S_n=(a_1+a_n)/(2)\cdot n\\a_1=3\\d=3\\a_n=99\\n=?\\\\a_n=a_1+(n-1)\cdot d\\99=3+(n-1)\cdot 3\\3n-3=96\\3n=99\\n=33\\\\S_(33)=(3+99)/(2)\cdot33=51\cdot33=1683`