Trigonometric function is often used in the stress analysis of building mechanics, such as wind load on building facade. If the direction of the load is not horizontal, but has an included angle of 15 degrees with the facade, then the force should be decomposed into horizontal and vertical directions; Of course, there are many forces acting on buildings, which need to be decomposed when analyzing them. There are also dynamic loads acting on buildings, such as the load when a train passes through a bridge, which also need to be expressed by trigonometric functions. Like earthquake loads, trigonometric functions are needed.