1) 3х^2-28х+9=0
2) 2х^2-8х+11=0
3) 16х^2-8х+1=0
4) 7х^2-21=0

1)3х^2-28х+9=0

D = b^2 - 4 * a * c = (-28)^2 - 4 * (3) * (9) = 676

x1 = (-b + sqrt(D)) / (2*a)

x2 = (-b - sqrt(D)) / (2*a)

x1 = 9

x2 = 1/3

2)2х^2-8х+11=0

D = b^2 - 4 * a * c = (-8)^2 - 4 * (2) * (11) = -24

                 ___

         I*\/ 6

x1 = 2 + -------

                 2  

                  ___

         I*\/ 6

x2 = 2 - -------

            2  

3)16х^2-8х+1=0

D = 0, то корень всего один.

x = -b/2a = --8/2/(16)

x = 1/4

4)7х^2-21=0

D = b^2 - 4 * a * c =

(0)^2 - 4 * (7) * (-21) = 588

             ___

x1 = \/ 3

               ___

x2 = -\/ 3