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# y=5+24x-9x^2-2x^3 Watch

1. Finding stationary points and drawing them

y=5+24x-9x^2-2x^3

dy/dx= 24-18x-6x^2

as dy/dx = 0 at stationary points

we get 24- 18x - 6x^2=0

divide by 6

4-3x-x^2=0

take x^2 to the other side

x^2+3x-4=0

(x+4)(x-1)=0

x=-4

x=1

sub x to find y values > original function y=5+24x-9x^2-2x^3

not showing that step too long

(1,18)

(-4,181)

dy/dx = d^2y/ dx^2

-18-12x

sub x values to find out whether stationary points are shaped max or min

sub x values > -18-12x

for x = -4 30>0 so min

for x = 1 30<0 so max

Am I right, because when I draw the graph I get a weird and wrong looking graph

y intercept is 5
2. ..........
3. http://www.wolframalpha.com/input/?i...B24x-9x^2-2x^3

Always a useful tool for visualising and checking your solutions

+rep if this helped!

oh and YES your answer is correct. just make sure to check your graph is the same as above then u nailed it!
4. yeh thats right, apart from a dodgey y calculation

you can always use octave and code like this to draw it out
Code:
x = -6:0.1:2;

y=5+24.*x-9.*x.^2-2.*x.^3;

plot(x,y)
5. I know what u lot are saying, but can u show me where i went wrong?
6. (Original post by Peace0fM1nd)
...
somebodies having a neg rep rampage in this thread :/
7. can someone show me where i went wrong?

I went wrong for -2(-4)^3 I got this as 128, when its -128

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