Question

Find m∠C. A. 16 B. 20 C. 18 D. 17

Answer 1

Explanation:

You have to use the Law of Cosines here to solve for C. For us the equation will take on the form

`c^2=a^2+b^2-2abcosC` for which we have all the values except C. Filling in:

`12^2=28^2+18^2-2(28)(18)cosC` which will simplify to

144 = 784 + 324 - 1008cosC. Simplify a bit more to

144 = 1108 - 1008cosC. Subtract 1108 from both sides to get

-964 = -1008cosC. Divide both sides by -1008 to get

.9563492063 = cosC

Press the 2nd button and then the cos button on your calculator. You should see this on your screen:

`cos^(-1)(`

After that open parenthesis, enter that long decimal and hit "enter". You should get that your angle is 16.99 which I imagine you would round to 17, which is choice D. If you did not get 16.99 but you got .29655, you are in radian mode and you need to be in degree mode. Hit "mode" and change it with your arrow key and hit "enter" and it's changed!