C2 maths help!

Find, in degrees, the value of Theta in the interval of 0<=Theta<360 for which:

2cos^2Theta – cosTheta – 1 = Sin^2Theta

I have solved this to get costheta= -2/3 and costheta= 1 but when putting it into my calculator I only get the answer of 131.8 and 0 is this this it?

Reply 1

Friendly Fatty

Question, are you in the first or second year?

Reply 2

TheConfusedMedic

21

Original post by Sukjeetsandhu

Find, in degrees, the value of Theta in the interval of 0<=Theta<360 for which:

2cos^2Theta – cosTheta – 1 = Sin^2Theta

I have solved this to get costheta= -2/3 and costheta= 1 but when putting it into my calculator I only get the answer of 131.8 and 0 is this this it?

No, there are 2 more solutions. If you sketch the graph of cosx for the given interval you should be able to see the other two solutions.
Alternatively, you can use the CAST diagram if you have been taught that in class (but this is just a simplification of the graphical approach).

graph of y = cosx:

Attachment not found

(edited 7 years ago)

Reply 3

will'o'wisp

3

Original post by Sukjeetsandhu

Find, in degrees, the value of Theta in the interval of 0<=Theta<360 for which:

2cos^2Theta – cosTheta – 1 = Sin^2Theta

I have solved this to get costheta= -2/3 and costheta= 1 but when putting it into my calculator I only get the answer of 131.8 and 0 is this this it?