Explanation:
We must prove that
cos²a(csc²a-cot²a) = cos²a
If we look at both sides, we can see that we have cos²a * something = cos²a. Therefore, if we can get that something to equal 1, we have our proof. In this case, that something is csc²a-cot²a. Using this information, we can work from within the parenthesis and go from there.
We can start by expanding the items in the parenthesis. Taking that csc(x) = 1/sin(x) and cot(a) = cos(x)/sin(x), we can say that
cos²a(csc²a-cot²a) = cos²a(1/sin²a - cos²a/sin²a). Because both items in the parenthesis have a denominator of sin²a, we can subtract cos²a from 1 to get
cos²a(1/sin²a - cos²a/sin²a)= cos²a((1-cos²a)/sin²a))
Next, we know that cos²a+sin²a=1, so 1-cos²a = sin²a. Plugging that in, we get
cos²a((1-cos²a)/sin²a)) = cos²a(sin²a/sin²a)
= cos²a(1)
= cos²a