Using direct variation

Which best explains why this triangle is or is not a right triangle?

Bring the fraction a/a-4 to a denominator of 16-a^2

Monitors manufactured by tsi electronics have life spans that have a normal distribution with a standard deviation of 1800 hours and a mean life span of 20,000 hours. if a monitor is selected at random, find the probability that the life span of the monitor will be more than 17,659 hours. round your answer to four decimal places.