NCERT Solutions
Class 10 Maths
Real Numbers

Ex. 1.2 Q3
Prove that the following are irrationals:
(i) 1/√2
(ii) 7√5
(iii) 6 + √2
(i) Let take that 1/√2 is a rational number.
So we can write this number as
1/√2 = a/b
Here a and b are two co prime number and b is not equal to 0
Multiply by √2 both sides we get
1 = (a√2)/b
Now multiply by b
b = a√2
divide by a, we get
b/a = √2
Here, a and b are integer, so b/a is a rational number. It means √2 should be a rational
number. But √2 is an irrational number. Hence, it is contradict the fact.
Hence, 1/√2 is an irrational number
(ii) Let take that 7√5 is a rational number.
So, we can write this number as
7√5 = a/b
Here a and b are two co prime number and b is not equal to 0
Divide by 7 we get,
√5 =a/7b
Here, a and b are integer, so a/7b is a rational number.
It means √5 should be a rational number but √5 is an irrational number.
So, it is contradict the fact.
Hence, 7√5 is an irrational number.
(iii) Let take that 6 + √2 is a rational number.
So, we can write this number as
6 + √2 = a/b
Here a and b are two co prime number and b is not equal to 0
Subtract 6 both side, we get
√2 = a/b – 6
√2 = (a - 6b)/b
Here ,a and b are integer, so (a - 6b)/b is a rational number.
It means √2 should be a rational number but √2 is an irrational number.
So, it is contradict the fact.
Hence, 6 + √2 is an irrational number.