Trigonometric Identities

Pythagorean: sin²θ+cos²θ=1, 1+tan²θ=sec²θ, 1+cot²θ=cosec²θ. Sum/difference: sin(A±B)=sinAcosB±cosAsinB. cos(A±B)=cosAcosB∓sinAsinB. tan(A+B)=(tanA+tanB)/(1-tanAtanB). Double angle: sin2A=2sinAcosA. cos2A=cos²A-sin²A=1-2sin²A=2cos²A-1.