slight optimization to c144.hw4.e06

c144.hw4.e06: Solve Quadratic Equation
* skip the square root of zero discriminant
* change type of discriminant (decimal -> double)
    Changed the type 'cause the precision is lost anyways
    (thanks to Math.Sqrt)
c144.hw4.e09-e10: HomeWork4 Exercises 09-10

E09 - First 100 Fibonacci Numbers
E10 - Sum of Sequence