Plotting the graph of this quadratic equation

[Guest]

Plot the following

y = 8 + 2x - x^2


Question: Is the concavity downwards? Working out is shown below...

y-intercept
let x be zero
y = (0) square - 2(0) - 8
= -8

x intercept
let y be zero
x square - 2x - 8 = 0
(x-4) (x+2)
x=4 x= -2

axis of symmetry:

4-2 / 2 = 1

y = 1 square - 2(1) - 8
= 1-2-8
= -9

vertex or turning point = (1, -9)
thanks !

 

[steve_b]

You have 2 sign errors in your vertex calculation. It should be +8 not -8 and +2 not -2, which gives the vertex (1,9). Also, it is "concave down".

Steve

 

[Guest]

thank you!

A very good answer