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Prove that 3 + 2sqrt5 is irrational - Mathematics

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Prove that 3 + 2`sqrt5` is irrational

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Solution

if possible let a = 3 + 2`sqrt5` be a rational number.

Squaring a2 = (3+2`sqrt5`)2

a2 = 29 +12`sqrt5`

`sqrt5` = `(a^2 - 29)/12` ........(1)

Since a is a rational number the expression `(a^2-29)/12` is also rational number.

⇒ `sqrt5` is a rational number.

This is a contradiction. Hence, 3 + 2`sqrt5` is irrational.

Hence Proved.

Concept: Concept of Irrational Numbers
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APPEARS IN

NCERT Class 10 Maths
Chapter 1 Real Numbers
Exercise 1.3 | Q 2 | Page 14
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