# What is Trigonometry

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“Tri” means three, “gon” means side and “metry” means measurement. Measurement of three sides is the Trigonometry. Hence in Trigonometry we deal with triangles.
• When two angles of a triangle were given then the third side of it can be calculated by using the theorem “Sum of all angles in a triangle is 180°”.
• When the length of two sides is given then the range of third side can be calculated.
• Here in most of the cases a right angled triangle is used.
Pythagoras theorem is c² = a² + b²
• All the trigonometric ratios (sin, cos, tan, cosec, sec, cot) are defined by using a right angled triangle.
• Some of the trigonometric identities are also defined.
• sin² a + cos² a = 1
• sec² a - tan² a = 1
• csc² a - cot² a = 1
• Sum and difference formulae:
• Sin ( x + y) = sin x cos y + cos x sin y
• Sin (x – y) = sin x cos y – cos x sin y
• Cos (x+ y) = cos x cos y – sin x sin y
• Cos (x – y) = cos x cos y + sin x sin y