Pre Calculus 11 - Trigonometry

Question

Find the two standard angles between 0° and 360° whose cosine value is -0.7. Round your answer to 1 decimal place

Any help at all with this question would be greatly appreciated!!

Thank you

Answer 1

Hi Declan V.,

Cosine is negative in quadrants 2 & 3. We can find the angle by taking the inverse cosine of -0.7. If I plug into my calculator:

cos^-1 (-0.7) = 134.2^o , which is in quadrant 2.

For quadrant 3 we take the inverse cosine of +0.7 and add 180^o

cos^-1(0.7) = 45.6^o

45.6^o + 180^o = 225.6^o

To check, make sure your calculator is in degree mode, then plug into your calculator cos(134.2) and cos(225.6) and see what you get.

Have fun! I hope this helps, Joe.