00:01
Okay, so we want to prove that the square of any rational number is rational.
00:06
So let's suppose that r is rational.
00:12
This is the first thing we're going to do.
00:14
Suppose that r is rational, because then we want to prove that it's square is rational.
00:19
So by definition, so i'll try to write this word for word.
00:24
By definition of rational, we have that r is equal to a over b for some, integer b with b not equal to zero so b is an integer sometimes you constrain it to be just positive but it doesn't need to be b is an integer and a is also an integer you should remember this so a is an integer also so by substitution r squared is equal to well if r is a over b then r squared is a over b squared is a over b squared and a over b squared is the same as a squared over b squared, right? so now since a and b are both integers, we have that.
01:27
So are the products, so are the products, a squared and not plus, a squared, and b squared.
01:44
So if a is an integer, then a times a is also an integer.
01:47
So imagine if a was 2, then a times a would be 4, which is an integer.
01:52
And b is also an integer...