# maximum point

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• October 28th 2010, 04:55 PM
Natasha1
maximum point
Find the coordinates of the maximum point on the curve y = -2xsquared + 6x + 3. Explain how you know it is a maximum?

f'(x) = -4 x + 6
so maximum point is when f'(x) = 0 so
-4x + 6 = 0
-4 x = -6
x = 1.5

Sustituing we get

y = 7.5

so (1.5, 7.5) is this correct?

How can I garanty it being the maximum point though?
• October 28th 2010, 05:01 PM
TheCoffeeMachine
Quote:

Originally Posted by Natasha1
so (1.5, 7.5) is this correct?

Yes. (Yes)
• October 28th 2010, 05:02 PM
Natasha1
Please explain how you know it is a maximum?

Moderator edit: Added "Please" and deleted excessive ?'s.
• October 28th 2010, 05:11 PM
TheCoffeeMachine
Quote:

Originally Posted by Natasha1
Explain how you know it is a maximum????

Say the function $f(x)$ has a stationary point at $\xi$. Then the second derivative test states that $\xi$ is a
maximum if $f''(\xi)<0$, and a minimum if $f''(\xi)>0$, and a possible point of inflexion if $f''(\xi) = 0$.
You have $f'(x) = -4 x + 6$, thus $f''(x) = -4$. If $x = 7.5$, we still have $f''(x) = -4 <0$.
• October 28th 2010, 05:23 PM
Natasha1
so the second derivative of f'(x) = -4 x + 6 would be

f''(x) = -4 which is < 0 and hence it is by definition the maximum point
• October 28th 2010, 05:25 PM
TheCoffeeMachine
Quote:

Originally Posted by Natasha1
so the second derivative of f'(x) = -4 x + 6 would be

f''(x) = -4 which is < 0 and hence it is by definition the maximum point

That's right.