Question

Find the sum of 2, 7, 12….… to 10 terms:
(a) 160
(b) 245
(c) 290
(d) 300

Answer 1

Hint: Notice that the terms of the series given in the question are in arithmetic progression with common difference and the first term equal to 5 and 2, respectively. So, directly use the formula of sum of an A.P. to get the answer. The sum of first r terms of an AP is given by $ {{S}_{r}}=\dfrac{r}{2}\left( 2a+\left( r-1 \right)d \right) $ , where a is the first term and d is the common difference of the AP.

Complete step-by-step answer:
Before starting with the solution, let us discuss what an A.P. is. A.P. stands for arithmetic progression and is defined as a sequence of numbers for which the difference of two consecutive terms is constant. The general term of an arithmetic progression is denoted by $ {{T}_{r}} $ , and sum till r terms is denoted by $ {{S}_{r}} $ .
 $ {{T}_{r}}=a+\left( r-1 \right)d $
 $ {{S}_{r}}=\dfrac{r}{2}\left( 2a+\left( r-1 \right)d \right)=\dfrac{r}{2}\left( a+l \right) $
Now moving to the series given in the question. We can see that the terms of the series given in the question are in A.P. with common difference equal to 5 and the first term of the A.P. being 2.
Now to find the sum of the A.P., we will use the formula $ {{S}_{r}}=\dfrac{r}{2}\left( 2a+\left( r-1 \right)d \right) $ . In the formula a is the first term, so a=2 and d is the common difference, d=5. Therefore, the sum of the series 2, 7, 12….… to 10 terms is:
 $ {{S}_{r}}=\dfrac{r}{2}\left( 2a+\left( r-1 \right)d \right) $
 $ \Rightarrow {{S}_{r}}=\dfrac{10}{2}\left( 2\times 2+5\left( 10-1 \right) \right) $
 $ \Rightarrow {{S}_{r}}=5\left( 4+5\times 9 \right) $
 $ \Rightarrow {{S}_{r}}=5\times 49=245 $
So, the correct answer is “Option B”.

Note: It is not always necessary that you can notice that the sequences are in the form of simple arithmetic progressions and draw results, but it is for sure that the terms of a sequence will have some order, and it depends on you how wisely you figure out the pattern. The suggested approach can be to try to find the general term of the sequence by observing the pattern of numbers appearing in the sequence and proceed. For the above question if you don’t know the formula of sum of r terms of the AP, you can directly add the 10 consecutive terms to get the answer.