Question

The law of cosines is a^2+b^2-2abcos(C). Find the value of 2abcos(C).

A. 37
B. -40
C. 40
D. 20

Answer 1

A. 37 i just took the quiz

Answer 2

`\bf \textit{Law of Cosines}\\ \quad \\ c^2 = {{ a}}^2+{{ b}}^2-(2{{ a}}{{ b}})cos(C)\\\\ -------------------------------\\\\ \begin{cases} c=2\\ a=4\\ b=5 \end{cases}\implies 2^2=4^2+5^2-(2ab)cos(C) \\\\\\ (2ab)cos(C)=4^2+5^2-2^2`

and surely you know how much that is.