Question

prove that 3+√5 is irrational

Answer 1

$Here \: is \: the \: answer \: of \: your \: question$

Prove = 3 + √5 is irrational number

→ Let us assume that 3 + √5 is a rational number.

Now,

3 + √5 = a ÷ b

[Here a and b are co-prime numbers]

√5 = a/b - 3

√5 = (a - 3b) ÷ b

Here, {(a - 3b) ÷ b} is a rational number.

But we know that √5 is a irrational number.

So, {(a - 3b)} is also a irrational number.

So, our assumption is wrong.

3 + √5 is a irrational number.

Hence, proved.

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How √5 is a irrational number.?

→ √5 = a ÷ b [a and b are co-prime numbers]

b√5 = a

Now, squaring on both side we get,

5b² = a² ........(1)

b² = a² ÷ 5

Here 5 divide a²

and 5 divide a also

Now,

a = 5c [Here c is any integer]

Squaring on both side

a² = 25c²

5b² = 25c² [From (1)]

b² = 5c²

c² = b² ÷ 5

Here 5 divide b²

and 5 divide b also

→ a and b both are co-prime numbers and 5 divide both of them.

So, √5 is a irrational number.

Hence, proved

Answer 2

Let, 3 + √5 be a rational number.

We know that, 3 is a rational number and
Also,

Rational - Rational = Rational

=> 3 + √5 - 3 = Rational

=> √5 = Rational

Since, √5 is an irrtaional number, so here our supposition contradicts.

Thus, we can say that 3 +√5 is an irrational number.