Question

Prove that ⅖ - V5 is irrational, given that V5 is irrational.​

Answer 1

(1) Let us assume that √5 is a rational number.

we know that the rational numbers are in the form of p/q form where p,q are intezers.                                                                                                          so, √5 = p/q

 p = √5q

we know that 'p' is a rational number. so √5 q must be rational since it equals to p

but it doesnt occurs with √5 since its not an intezer

therefore, p =/= √5q

this contradicts the fact that √5 is an irrational number

hence our assumption is wrong and √5 is an irrational number.

(2)Let us assume that 5+2√3 is rational

5+2√3 = p/q ( where p and q are co prime)

2√3 = p/q-5

2√3 = p-5q/q

√3 = p-5q/2q

now p , 5 , 2 and q are integers  

∴ p-5q/2q is rational

∴ √3 is rational

but we know that √3 is irrational . This is a contradiction which has arisen due to our wrong assumption.

∴ 5+2√3 is irrational