Pythagorean trigonometric identity
（重定向自Pythagorean identity）
![Similar right triangles showing sine and cosine of angle θ](/Images/godic/202502/05/Trig_functions.svg0506.png")
![Similar right triangles illustrating the tangent and secant trigonometric functions.](/Images/godic/202502/05/Trig_functions2.svg0506.png")
![Point P(x,y) on the circle of unit radius at an obtuse angle θ > π/2](/Images/godic/202502/05/Trig_functions_on_unit_circle0507.PNG")
![Sine function on unit circle (top) and its graph (bottom)](/Images/godic/202502/05/Periodic_sine0507.PNG")
The Pythagorean trigonometric identity is a trigonometric identity expressing the Pythagorean theorem in terms of trigonometric functions. Along with the sum-of-angles formulae, it is one of the basic relations between the sine and cosine functions.