Answer: Choice D
s^2 = 9^2 + 10^2 - 2(9)(10)cos(100)
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Explanation:
Consider the pythaorean theorem
a^2 + b^2 = c^2
which is the same as saying
c^2 = a^2 + b^2
If we tack on "-2ab*cos(C)", then we end up with
c^2 = a^2 + b^2 - 2ab*cos(C)
which is one form of the law of cosines. The side c is opposite angle C, and the two are on opposite sides of the equation. The 'a's and 'b's are on the right side with the angle C.
Note if C = 90 degrees, then cos(C) = cos(90) = 0 which means the whole "-2ab*cos(C)" term goes to 0 and we'd be left with the pythagorean theorem again.
In this case, we replace c with s, replace 'a' with 9, replace b with 10, and C with 100.
