need help (math)?

Question

Find the minimum or maximum value of the quadratic function
y=5x^2-9x-1

Answer 1

minimum/maximum occur when y' = 0

find the derivative, y' = 10x - 9

set equal to zero, 10x - 9 = 0

solve for x, x = 9/10

at x = 9/10 you have a min/max

Answer 2

y = 5x² - 9x - 1
min when dy/dx = 0
dy/dx = 10x - 9 = 0
10x = 9
x = 9/10

y at min = 5(9/10)² - 9(9/10) - 1 =
5(81/100) - 81/10 - 1 =
405/100 - 810/100 - 100/100 =
-505/100 = -5.05

Answer 3

10x-9=0
x=9/10

y=5,05