Question

extreme value of y=2x^2-8x+9

Answer 1

Do you mean find the maximum or minimum value of y, defined by
\[y=2x^2-8x+9 \]?

if so, see
http://www.khanacademy.org/math/differential-calculus/derivative_applications/critical_points_graphing/v/identifying-minima-and-maxima-for-x-3---12x---5

Answer 2

or the introduction to the ideas:
http://www.khanacademy.org/math/differential-calculus/derivative_applications/critical_points_graphing/v/minima--maxima-and-critical-points

Answer 3

thank you:)