Answer:
D) cos^-1 (f/e)
Step-by-step explanation:
Since you only know two sides, DE = f, and DF = e, you need to use cosine to find the measure of ∠D.
Let x represent m∠D
cos x = f/e
To find x, take the inverse of cos, so:
cos^-1 (f/e) = x

Given that
and
, it follows that


###

When
, we have

The slope of the line tangent to
at (0, 3) has slope
,

Then the tangent line at this point has equation

###
Differentiating both sides of

with respect to
yields



On this curve, when
we have

(ignoring the negative solution because we don't care about it)
The tangent to this curve at the point
has slope
. This tangent line is horizontal when its slope is 0. This happens for

and when
, there is a horizontal tangent line to the curve at the point (2, 8).
If two plus two is four therefore two plus three is in fact five.
The radius is 12 because 12^2 equals 144