If the `n^(t h)` term of an A.P., is `(2n+1),` then the sum of its first three terms is (a) 6n+3 (b) 15 (c) 12 (d) 21

Explanation

nth term of the AP, a_{n}=2 n+1

therefore, First term,` a_{1}=2 times 1+1=2+1=3`

Second term, `a_{2}=2 times 2+1=4+1=5`
Third term,` a_{3}=2 times 3+1=6+1=7`

therefore Sum of the first three terms` a_{1}+a_{2}+a_{3}=3+5+7=15`