If a triangle is inscribed in a circle so that one of the triangle's sides is a diameter of the circle, what is the greatest area that the triangle can have in terms of the radius, r, of the circle?

Charlie is at an amusement park that has 18 different rides. each ride costs $1.75. the function c(r) = 1.75r represents the total amount of money that charlie will spend if he goes on r rides, but he never goes on the same one more than once. what domain and range are reasonable for the function?