twenty - one dots are evenly spaced on the circumference of a circle . how many combinations of three dots can we make from these 21 dots that do not form an equilateral triangle ?
the total number of ways we can choose three dots is 21 c 3 = 1330 . we can form seven equilateral triangles from these twenty - one dots . there are 1330 - 7 = 1323 combinations which do not form an equilateral triangle . the answer is e .
subtract(divide(divide(multiply(multiply(21, subtract(21, const_1)), subtract(subtract(21, const_1), const_1)), const_3), const_2), add(const_3, const_4))