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September 21, 2014

September 21, 2014

Posted by **Hannah** on Saturday, February 26, 2011 at 2:57pm.

I was going to take the derivative of the equation and set it equal to 0 and then plug it back in but it didn't work. The answer is 300 but I do not know how to get this.

- Math -
**MathMate**, Saturday, February 26, 2011 at 3:01pmYour approach actually works, you might have made an inversion in the sign or something.

P(x)=-x²+40x-100

P'(x)=-2x+40=0 => x= 20

P(20)=-20²+40*20-100=300 QED

- Math -
**Hannah**, Saturday, February 26, 2011 at 3:05pmoh ok I did I got x=-20 instead of 20. Thank you!!

- Math -
**Hannah**, Saturday, February 26, 2011 at 3:09pmbut I still do not see how you got 300.

-20(2)=400 + 40(20)=800 - 100 = 1100?

- Math -
**Ryan**, Saturday, February 26, 2011 at 3:26pmVertex formula= -b/2a

= -40/-2 = 20

P(-20)=-(-20^2)+(40*-20)-100

P(-20)=-(400)+(800)-100

P(-20)=300

- Math -
**Ryan**, Saturday, February 26, 2011 at 3:28pmMessed up my signs alittle bit should read

P(20)=-(20^2)+(40*20)-100

P(20)=-(400)+(800)-100

P(20)=300

- Math -
**MathMate**, Saturday, February 26, 2011 at 3:32pmExactly...

P(20)=-(400)+(800)-100 =300

- Math -
**Hannah**, Saturday, February 26, 2011 at 3:33pmoh ok so its -400. Thanks I see now

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