12.B

13.B

14. Positive and negative root sign 2

15.x- 1 x(x- 1)/2 x(x- 1)/2 = 28 x^2-x=56 x 1 = 8，x2=-7 x=8 8

16. solution: take X 2 as y.

y^2-y-6=0

y^2-y+( 1/2)^2=6+ 1/4

(y- 1/2)^2=25/4

Y- 1/2= plus or minus 5/2.

∴y 1=3,y2=-2

When y=3

x^2=3

X= plus or minus three.

When y=-2

x^2=-2

X has no solution

The solution of the original equation is x 1= root number three, x2 =- root number three.

Please write the format according to the teacher's instructions. If you don't understand, keep asking. Sorry, that's all I know.