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Question 27 (OR 1st question) - CBSE Class 10 Sample Paper for 2020 Boards - Maths Standard - Solutions of Sample Papers for Class 10 Boards

Last updated at Dec. 16, 2024 by Teachoo

Given that √5 is irrational, prove that 2√5 − 3 is an irrational number.

 

Note : This is similar to Ex 1.3, 2 of NCERT – Chapter 1 Class 10

Check the answer here https://www.teachoo.com/1472/489/Ex-1.3--2---Prove-that-3---2-root-5-is-irrational/category/Ex-1.3/

Transcript

Question 27 (OR 1st question) Given that √5 is irrational, prove that 2√5 − 3 is an irrational number. We have to prove 2√5 – 3 is irrational Let us assume the opposite, i.e., 2√5 – 3 is rational Hence, 2√5 – 3 can be written in the form 𝑎/𝑏 where a and b are co-prime and b ≠ 0 Hence, 2√5 – 3 = 𝑎/𝑏 2√5 = 𝑎/𝑏 + 3 2√5 = (𝑎 + 3𝑏)/𝑏 √5 = 1/2 × (𝑎 + 3𝑏)/𝑏 √5 = (𝑎 + 3𝑏)/2𝑏 Here, (𝑎 + 3𝑏)/2𝑏 is a rational number But √5 is irrational Since, Rational ≠ Irrational This is a contradiction ∴ Our assumption is incorrect Hence, 2√5 – 3 is irrational Hence proved
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Davneet Singh

Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 14 years. He provides courses for Maths, Science and Computer Science at Teachoo