What is the Formula of Cos(A – B) ?

In this post you will learn what is the formula for cos (A – B) with examples.

Cos (A – B) Formula :

The formula of cos(A – B) is cos A cos B + sin A sin B.

Example : If sin A = \(3\over 5\) and cos B = \(9\over 41\), find the value of cos (A – B).

Solution : We have,

sin A = \(3\over 5\) and cos B = \(9\over 41\)

\(\therefore\)  cos A = \(\sqrt{1 – sin^2 A}\)  and  sin B = \(\sqrt{1 – cos^2 B}\)

\(\implies\)  cos A = \(\sqrt{1 – {9\over 25}}\) = \(4\over 5\)  and  sin B = \(\sqrt{1 – {81\over 1681}}\) = \(40\over 41\)

Now, By using above formula,

cos (A – B) = cos A cos B – sin A sin B

= \(4\over 5\) \(\times\) \(9\over 41\) + \(3\over 5\) \(\times\) \(40\over 41\) = \(156\over 205\)

Example : Find the value of cos 15.

Solution : cos 15 = sin (45 – 30)

By using above formula,

cos (45 – 30) = cos 45 cos 30 – sin 45 sin 30

\(\implies\) cos 15 = \(1\over \sqrt{2}\) \(\times\) \(\sqrt{3}\over 2\) + \(1\over \sqrt{2}\) \(\times\) \(1\over 2\)

\(\implies\) cos 15 = \(\sqrt{3} + 1\over 2\sqrt{2}\)

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