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Prove root 5 is rational number

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Prove root 5 is rational number
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? Royal Thakur ? 5 years, 8 months ago

To prove that √5 is irrational number  Let us assume that √5 is rational  Then √5 =   (a and b are co primes, with only 1 common factor and b≠0)  ⇒ √5 =   (cross multiply)  ⇒ a = √5b  ⇒ a² = 5b² -------> α ⇒ 5/a²  (by theorem if p divides q then p can also divide q²)  ⇒ 5/a ----> 1  ⇒ a = 5c  (squaring on both sides)  ⇒ a² = 25c² ----> β  From equations α and β  ⇒ 5b² = 25c² ⇒ b² = 5c²  ⇒ 5/b²  (again by theorem)  ⇒ 5/b-------> 2  we know that a and b are co-primes having only 1 common factor but from 1 and 2 we can that it is wrong.  This contradiction arises because we assumed that √5 is a rational number  ∴ our assumption is wrong  ∴ √5 is irrational number
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