Trigonometric identities

Trigonometric identities

 Wherever ‘square’ appears think of using the identities

•  sin^2(x) + cos^2(x) = 1
• tan^2(x) + 1 = sec^2(x)
• cot^2(x) + 1 = csc^2(x)
• tan(x y) = (tan x tan y) / (1 tan x tan y)
• sin(2x) = 2 sin x cos x
• cos(2x) = cos^{^2}(x) – sin^{^2}(x) = 2 cos^{^2}(x) – 1 = 1 – 2 sin^{^2}(x)
• tan(2x) = 2 tan(x) / (1 – tan^{^2}(x))
• sin^{^2}(x) = 1/2 – 1/2 cos(2x)
• cos^{^2}(x) = 1/2 + 1/2 cos(2x)
• sin x – sin y = 2 sin( (x – y)/2 ) cos( (x + y)/2 )
• cos x – cos y = -2 sin( (x – y)/2 ) sin( (x + y)/2 )