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    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
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    ..........
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    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!
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    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)
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    I know what u lot are saying, but can u show me where i went wrong?
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    (Original post by Peace0fM1nd)
    ...
    somebodies having a neg rep rampage in this thread :/
    • Thread Starter
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    can someone show me where i went wrong?
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    I get the answer

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

    my bad =P
 
 
 
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