Asked/Added Questions From NCERT:

Question 1. Asked on :09 July 2019:07:05:00 PM

If the sum of n^th terms of an A.P. is given by 4n² - 7n. Find
(i) Sum of 23 terms
(ii) 15^th term
(iii) n^th term

-Added by ATP Admin Mathematics » Arithmetic Progressions

Answer:

Master Mind

Given:S_n = 4n² - 7n

Step by step explanation:

(i) Sum of 23 terms

S_n = 4n² - 7n ............. (i)

Replace n by 23 we have,

S₂₃ = 4(23)² - 7(23)

= 4 × 529 - 161

= 2116 - 161

= 1955 Answer

(ii) 15^th term

a₁₅ = S₁₅ - S₁₄

S₁₅ = 4(15)² - 7(15) = 4 × 225 - 105 = 900 - 105 = 795

S₁₄ = 4(14)² - 7(14) = 4 × 196 - 98 = 784 - 98 = 686

Now, a₁₅ = S₁₅ - S₁₄

= 795 - 686

= 109 Answer

(iii) n^th term

a_n = S_n - S_n-1

Given, S_n = 4n² - 7n

Replace n by n-1 we have

S_n-1 = 4(n-1)² - 7(n-1)

= 4(n² - 2n + 1) - 7n + 7

= 4n² - 8n + 4 - 7n + 7

= 4n² - 15n + 11 ............. (ii)

Now Applying equation (i) and (ii) in formula a_n = S_n - S_n-1

a_n = (4n² - 7n) - (4n² - 15n + 11)

= 4n² - 7n - 4n² + 15n - 11

= 8n - 11

Therefore, a_n = 8n - 11 Answer

*******************************

-Answered by Master Mind On 11 July 2019:09:42:19 PM(2123Average Rating Based on rating)

You can see here all the solutions of this question by various user for NCERT Solutions. We hope this try will help you in your study and performance.

This Solution may be usefull for your practice and CBSE Exams or All label exams of secondory examination. These solutions or answers are user based solution which may be or not may be by expert but you have to use this at your own understanding of your syllabus.

What do you have in your Mind....

Ask Your Question? From your text book.

Our Expert Team reply with answer soon.

Ask Your Question

* Now You can earn points on every asked question and Answer by you. This points make you a valuable user on this forum. This facility is only available for registered user and educators.

Next moment you answer is ready .... go ahead ...