Is It Irrational?
Here we look at whether a square root is irrational ... or not!
Rational Numbers
A "Rational" Number can be written as a "Ratio", or fraction.
Example: 1.5 is rational, because it can be written as the ratio 3/2
Example: 7 is rational, because it can be written as the ratio 7/1
Example 0.317 is rational, because it can be written as the ratio 317/1000
But some numbers cannot be written as a ratio!
They are called irrational (meaning "not rational" instead of "crazy!")
The Square Root of 2
The square root of 2 is irrational. How do I know? Let me explain ...
Squaring a Rational Number
First, let us see what happens when we square a rational number:
If the rational number is a/b, then that becomes a2/b2 when squared.
Example:
(34)2 = 3242
Notice that the exponent is 2, which is an even number.
But to do this properly we should really break the numbers down into their prime factors (any whole number above 1 is prime or can be made by multiplying prime numbers):
Example:
(34)2 = (32×2)2 = 3224
Notice that the exponents are still even numbers. The 3 has an exponent of 2 (32) and the 2 has an exponent of 4 (24).
In some cases we may need to simplify the fraction:
Example: (1690)2
Firstly: 16 = 2×2×2×2 = 24, and 90 = 2×3×3×5 = 2×32×5
(1690)2 = (242×32×5)2
= (2332×5)2
= 2634×52
But one thing becomes obvious: every exponent is an even number!
So we can see that when we square a rational number, the result is made up of prime numbers whose exponents are all even numbers.
When we square a rational number, each prime factor has an even exponent.
Back to 2
Now, let us look at the number 2: could this have come about by squaring a rational number?
As a fraction, 2 is 21
Which is 2111 that has odd exponents!
But we want even exponents (so its square root will be rational)
We could write 1 as 12 (so it has an even exponent), and then we have:
2 = 2112
But there is still an odd exponent (on the 2).
We can simplify the whole thing to 21, but still an odd exponent.
We could try things like 2 = 42 = 2221 but we still cannot get rid of an odd exponent.
Oh no, there is always an odd exponent.
So 2 could not have been made by squaring a rational number!
So its square root must be irrational.
In other words: whatever value that was squared to make 2 (ie the square root of 2) cannot be a rational number, so must be irrational.
Note: for another proof check out Euclid's Proof that Square Root of 2 is Irrational.
Try Some More Numbers
How about square root of 3?
3 is 3/1 = 31
But the 3 has an exponent of 1, so 3 could not have been made by squaring a rational number, either.
The square root of 3 is irrational
How about square root of 4?
4 is 4/1 = 22
Yes! The exponent is an even number! So 4 can be made by squaring a rational number.
The square root of 4 is rational
This idea can also be extended to cube roots, etc.
Conclusion
To find if the square root of a number is irrational or not, check to see if its prime factors all have even exponents.
It also shows us there must be irrational numbers (such as the square root of two) ... in case we ever doubted it!