Question 1010813

Express cos 90- cos 50 as a product containing only sines and/or cosines 
<pre>Difference of two angles formula: cos (A - B) = cos A cos B + sin A sin B
cos (90 - 50) = cos 90 cos 50 + sin 90 sin 50
cos (90 - 50) = 0 * cos 50 + 1 * sin 50
cos (90 - 50) = 0 + sin 50
{{{highlight_green(cos (90 - 50) = sin 50)}}}

OR

<u>Shortcut</u>
sin and cos are COFUNCTIONS, so cos (90 - 50) = cos 40 = sin (90 - 40), or sin 50