How do you find the maximum and minimum point in a graph?

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For example, when the question is "use the graph to find an estimate for the minimum value of Y" do you just look at the lowest value of Y on the graph or do you do something else?

Posted Sun 3rd March, 2013 @ 09:05 by Flaka Prekazi

5 Answers

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Yeah, I assume so.

Answered Tue 5th March, 2013 @ 21:29 by Hoosierette
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If you have the equation for the line, which you should do, you differentiate it, make it equal to zero, then solve to find the point.

For example if you have the graph y = x(2) + 7x + 5          (2) is squared


dy/dx = 2x + 7

0 = 2x + 7

x = -3.5

You then insert this into the original equation to find y:

y = -3.5(2) + 7 * -3.5 + 5

y = -7.25

Minimum point is (-3.5, -7.25)

You know it is a minimum because it is a positive x(2), if it is a negative x(2) then it is a maximum point.

Hope I have been of assistance to you :)

Answered Thu 4th April, 2013 @ 18:15 by CoolKid
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I thought you had to differentiate it twice? Differentiating once is to find gradients and differentiating twice is maximum and minimum points?

Answered Thu 4th April, 2013 @ 19:47 by Kate Westall
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Differentiating once works but can be harder to use at a higher level, at my level I find differentiating once much simpler, but you can differentiate twice and you get the second derivative and stuff however I believe you need to have an x value to work out the turning points? Not sure, my notes on it aren't very good so sorry I can't be of more help on that.

Answered Thu 4th April, 2013 @ 21:59 by CoolKid
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The function of differentiating twice is to determine whether the point is Local maximum or Local minimum. This is not neccessary on a Quadratic graph because it either has a Local maximum Or a Local minimum, not both. Like "CoolKid" said, if the equation of the line is  positive x^2             ( y= x^2 + 7x +5 ), then the line has a Local minium, the lowst point of the curve on the y axis where the gradient = 0. If the the equation of the line is negative x^2 (y= -x^2 +7x +5), then the line has a Local maximum, the highest point of the curve on the y axis where the gradient = 0. 

We use the second derivative when dealing with a cubic equation. This is because in a cubic, there is both a Local maximum and a Local minimum. After factorising the first derivative you will have two  x values. By substituting both these  x values into the second derivative we can determine if they are the x co-ordinate of a Local maximum or a Local minimum. If the value acheived is positive, the point on the graph will be a Local minimum. If the value acheived is negative, the point ont the graph will be a Local maximum

Now you have determined which point will be a Local maximum and Local minimum, all you have to do is substitute the corresponding x co-ordiante into the original equation to find the  y value. With both the Local maximum and Local minimum you can now plot your cubic graph.

Answered Sat 6th April, 2013 @ 15:11 by Alex