Question

cos (pi/4+pi/6)

Answer 1

\(\Large\color{blue}{ \sf π=180° }\) Therefore, \(\Large\color{blue}{ \sf π/4=180/4=45° }\) and \(\Large\color{blue}{ \sf π/4=180/6=30° }\)

SO, lets RE-WRITE what you got. Instead of \(\Large\color{blue}{ \sf cos (π/4+π/6) }\), you will NOW get \(\Large\color{blue}{ \sf cos (45° +30°) }\)


The formula below says that \(\Large\color{red}{ \sf cos(A+B)=cos A ~cos B - sin A~ sin B }\) http://www.ies-math.com/math/java/trig/kahote/kahote.html


Let plug in numbers
\(\Large\color{blue}{ \sf cos(45°+30°)=cos 45° ~cos 30° - sin 45°~ sin 30° }\)

we need to find each

\(\Large\color{blue}{ \sf cos 45° =?}\)

\(\Large\color{blue}{ \sf sin 45°=? }\)

\(\Large\color{blue}{ \sf cos 30°=? }\)

\(\Large\color{blue}{ \sf sin 30°=? }\)

you can refer to http://openstudy.com/users/solomonzelman#/updates/5327a134e4b0ba8d4c41f6ba

or to any other trigonometric table better than that awful one.

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