In an equilateral triangle, the length of each side is not equal, so a method is needed to calculate the side length and angle of the triangle. The side length formula of equilateral triangle, also known as cosine theorem, provides a method to calculate the side length and angle of triangle. The sum of squares of any two sides in a triangle minus the product of these two sides is proportional to the cosine of the included angle between these two sides. This formula can be used to calculate any side length of a triangle, as long as the included angle between the other two side lengths is known. This formula is widely used in mathematics, physics and engineering, and it is one of the important tools to solve the equilateral triangle problem.