2x3 − 5x + 39Then x is algebraic.
Because all conditions are met:
- 2x3 − 5x + 39 is a non-zero polynomial (a polynomial which is not just "0")
- x is a root (i.e. x gives the result of zero for the function 2x3 − 5x + 39)
- the coefficients (the numbers 2, −5 and 39) are rational numbers
Let's find an algebraic number:
Example: 2x3 − 5x + 39
We need to find the value of x where 2x3 − 5x + 39 is equal to 0
Well x = −3 works, because 2(−3)3 − 5(−3) + 39 = −54+15+39 = 0
so −3 is an Algebraic Number
Let's try another polynomial (remember: the coefficients must be rational).
Example: 2x3 − ¼ = 0
The coefficients are 2 and −¼, both rational numbers.
And x = 0.5, because 2(0.5)3 − ¼ = 0
so 0.5 is an Algebraic Number
In fact most numbers we use daily are algebraic.
Not Algebraic? Then Transcendental!
When a number is not algebraic, it is called transcendental.
What about the square root of 2?
Example: is √2 (the square root of 2) algebraic or transcendental?
√2 is a solution to x2 − 2 = 0, so it is algebraic (and not transcendental).
All algebraic numbers are computable and so they are definable.
The set of algebraic numbers is countable. Put simply, the list of whole numbers is "countable", and you can arrange the algebraic numbers in a 1-to-1 manner with whole numbers, so they are also countable.
The imaginary number i is algebraic (it is the solution to x2 + 1 = 0).
All rational numbers are algebraic, but an irrational number may or may not be algebraic.