show that equilateral triangle with sides of length r has area (square root of 3)/(4) multiplied by r squared?

show that equilateral triangle with sides of length r has area (square root of 3)/(4) multiplied by r squared? https://www.studyhelpus.com/Questions & Answers/show-that-equilateral-triangle-with-sides-of-length-r-has-area-square-root-of-3-4-multiplied-by-r-squared/