39, it is like the fibonacci sequence you take the 2 previous numbers and add them together to get the next number.
It's a value you should probably memorize:

You can derive it using some trigonometric identities, other known values of cosine, and properties of the cosine function. For example, using the double angle identity for cosine:

If
, then

and you probably know that
, so

When we take the square root, we should take the positive root because
whenever
:

0024r8 is the answer of your problem.
20
Explanation:
20 + 4 = 24. Therefore 24-20=4
Answer:
b^8
Step-by-step explanation:
When multiplying exponents with the same base, we can add the exponents
b^2 * b^6
b^ (2+6)
b^8