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(13r-a)^2?

Just think of it that it is like this:

13-squared x r-squared -a-squared

169 x r-squared a-squared

=169r^2+a^2

Now, WHY DID -a-squared become positive a-squared?

Answer: Given -3-squared:

We all know that -3-squared is 9.

But, it is just the same with 3-squared and both of them yields 9.

So, simple, we just simplified it, done!

Just think of it that it is like this:

13-squared x r-squared -a-squared

169 x r-squared a-squared

=169r^2+a^2

Now, WHY DID -a-squared become positive a-squared?

Answer: Given -3-squared:

We all know that -3-squared is 9.

But, it is just the same with 3-squared and both of them yields 9.

So, simple, we just simplified it, done!