When deriving the quadratic formula by completing the square, what expression can be added to both sides of the equation
create a perfect square trinomial?
x2 + b/a x + _ = -c/a + _

Astudent must have an average on five test that is greater than it equal to 80% but less than 90% to receive a final grade of b. devon's greades on the first four test were 78% 62% 91% and 80% what range if grades on the fifth test would give him a b in the course? ( assuming the highest grade is 100%)