Question

Find the sum of the first 22 terms of an AP in which $d = 7$ and ${22^{nd}}$ term is 149.

Answer 1

Use the formula of Arithmetic progression sequence for the nth terms that is ${a_n} = a + \left( {n - 1} \right)d$ where, a initial term of the AP and d is the common difference of successive numbers. Calculate the value of a. We use the formula of the sum of n terms in Arithmetic progression that is ${S_n} = \dfrac{n}{2}\left[ {2a + \left( {n - 1} \right)d} \right]$. Calculate the sum of the AP, ${S_n}$.

Complete step by step solution:
Given data: The ${22^{nd}}$ term that is given for an arithmetic progression is 149.
Common difference is $d = 7$
Now, we know about the Arithmetic progression sequence for the nth terms is given by the following expression:
${a_n} = a + \left( {n - 1} \right)d$
Here, the first term of the arithmetic progression sequence is $a$.
Now, calculate the value of $a$. Substitute the value of d = 7,n = 22 and ${a_n} = 149$ in ${a_n} = a + \left( {n - 1} \right)d$.
149 = a + (22- 1)7
149 = a + 147
a = 149 - 147
= 2
Now, we know about the formula of the sum of n terms in Arithmetic progression is given by the following expression:

${S_n} = \dfrac{n}{2}\left[ {2a + \left( {n - 1} \right)d} \right]$

Simplify the above equation by substituting ${a_n} = a + \left( {n - 1} \right)d$.

${S_n} = \dfrac{n}{2}\left[ {a + {a_n}} \right]$

Now, calculate the value of ${S_n}$ by substituting $n = 23$, $a = 2$ and $a_n = 149$ in the expression for the sum of the Arithmetic progression ${S_n} = \dfrac{n}{2}\left[ {a + {a_n}} \right]$.
${S_{22}} = \dfrac{{22}}{2}\left[ {2 + 149} \right]\\\ = 11\left[ {151} \right]\\\ = 1,661$
Hence, the sum of the first 22 terms of an Arithmetic progression is ${S_{22}} = 1,661$.

Note: The general equation of the Arithmetic progression is $a,a + d,a + 2d,a + 3d,...$, where a is initial term of the AP and d is the common difference of successive numbers. Make sure use the formula of the sum of n terms in Arithmetic progression that is ${S_n} = \dfrac{n}{2}\left[ {2a + \left( {n - 1} \right)d} \right]$ and use the Arithmetic progression sequence for the nth terms that is ${a_n} = a + \left( {n - 1} \right)d$.