Question

A student draws a triangle with a perimeter of 36cm. The student says the longest length is 18cm. How do you know the student is wrong?

Answer 1

The length of the first two sides of a triangle must be greater than the length of the last side.  If the longest length were 18, the first two sides would be too short.  36-18=18, 18 is equal not greater than 18 which means the sum of the first two sides are too short.