How to evaluate for cosine using the sum and difference identities

👉 Learn how to evaluate the cosine of an angle in radians using the sum/difference formulas. To do this, we first express the given angle as a sum or a difference of two (easy to evaluate) angles, then we use the unit circle and the Pythagoras theorem to identify the angles and obtain all the needed trigonometric function values of the angles. When we know the trigonometric function values of the two angles to be added or subtracted, we can apply the sum and difference formulas to evaluate the cosine of the given angle.

Organized Videos:
✅ Sum and Difference Formulas
✅ Evaluate Sum and Difference Formulas from a Triangle
✅ Simplify an Expression using Sum and Difference Formulas
✅ Write the Expression as a single function | Sum and Difference Formulas
✅ Verify Identities using Sum and Difference Identities
✅ Evaluate Tangent using Sum and Difference Formulas
✅ Evaluate Cosine using Sum and Difference Formulas
✅ Evaluate Sine using Sum and Difference of Two Angles
✅ Solve Equations using Sum and Difference Formulas

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[herbcruz]

I'm just wondering. Would it be easier to first use the fact that the cosine function is even (because cos(-x) = cos(x)), so cos(-15 degrees) = cos(15 degrees). Then use the cos((theta)/2) formula (Set (theta)/2 degrees = 15 degrees, which implies that theta = 30 degrees), and take the positive square root (in the cos((theta)/2) identity), since cos(15 degrees) is positive?

[vaishnavisawant]

Sin 2 ^/cos 2 ^+cos 2^/sin 2^/cos2^=sec2^-cosec2^-2. Prove L.H.S = R.H.S
sir, plz prove the above sum....