Tell whether the following statements is formal or non-formal. Write F if your answer is formal and NF if it is non-formal on the space provided before each item. 1. An acute triangle is a triangle that all included angles is less than 90 degrees. 2. The diagonal of a rhombus is perpendicular to each other. 3. A number is an even number if and only if n must be squared. 4. A number n is an odd number if and only if n = 2k + 1 where k is any integer. 5. An odd number raised to a third power is always an odd number.