If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Proof: cos(a+b) = (cos a)(cos b) - (sin a)(sin b)

This original Khan Academy video was translated into isiXhosa by Lwazi Pambuka. The translation project was made possible by ClickMaths: www.clickmaths.org