9.1

12: The hypotenuse is c.

15: It’s a right triangle if the Pythagorean theorem works for its side lengths.

19: (3√5)² ＝ 3² times (√5)².

31: The area of a triangle is half of its base times its height.

35: Sketch a right triangle where the hypotenuse is from one of the two points to the other.

45: Multiply the numerator and denominator by √3 to make the denominator a whole number.