Idea of Trigonometry Icon
It is a right triangle with the base on the bottom and a perpendicular side on the right. There is a square in the bottom-right corner, indicating that it is a right triangle. There is an arc between the hypotenuse and the bottom base, near the bottom-left corner. Each of the 3 corners has a small dot on it.
This page shows the different variations of the Trigonometry from an Icons8 icon pack. Feel free to resize and recolor the icon, and then download the icon in PNG format for free.