You should write a 1 because I f what you add is 11, 12, 13, 14, 15, 16, 17, 18, or 19 there is a 1 in the front of the numbers so you must put a 1 but if the number was 76 the number in the front is 7 so you would put 7 on the tens column.
Biased sample of what? I need more context :)
ANSWER
By simplifying the left hand side using the Pythagorean Identity.
EXPLANATION
The given identity is

Take the left hand side and simplify to get the right hand side.

Collect LCM for the denominators.

Recall the Pythagorean Identity.

This implies that:

We substitute this to get,


Discrete domains and continuous domains are both sets. However a discrete domain contains a finite number of elements and continuous can contain and infinite number of elements.