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Fundamental trigonometric identities, aka trig identities or trigo identities, are equations involving trigonometric functions that hold true for any value you substitute into their variables. These ...
This uses the trigonometry sum of angles formula and breaks the cosine into a function of sines and cosine with angles that we know the answer for (45° and 30°).
Prove the formula for the cosine of the sum of two angles; and deduce the formulas for the consine of the double of an angle and the cosine of the half of an angle. Wikimedia Commons ...
Revise trigonometric ratios of sine, cosine and tangent and calculate angles in right-angled triangles with this Bitesize GCSE Maths Edexcel guide.