can i have more help please? thank you:)

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how would you use a graph to solve

x (cubed) + 6x (squared) -19x + 84 = 0?

i can do it using the formula but not on a graph...please help!

thank you:)

Posted Sun 17th June, 2012 @ 11:50 by AmyPond
Edited by AmyPond on Sun 17th June, 2012 @ 12:01

9 Answers

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I would draw y=x^3+6x^2-19x+84 on the graph and then draw the line y=0 and the points of intersection is the answer for x.

Answered Sun 17th June, 2012 @ 12:58 by Dilly
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How do you draw it on a graph? Substitute numbers in? Thanks:)

Answered Sun 17th June, 2012 @ 13:22 by AmyPond
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Yeah I would work out what y was for x from maybe -5 to 5 (or may -4 to 4 might be enough) by substituting these into the equation.

Answered Sun 17th June, 2012 @ 13:24 by Dilly
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sorry, im really confused- how would you get y values out?

Answered Sun 17th June, 2012 @ 13:32 by AmyPond
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Okay maybe I'm just not explaining it properly :) so...

To work out the y value when x = 5 (substitute x=5 into the equation)

you would do y= 5^3+6*5^2-19*5+84

Then do that for all x values between 5 and -5 and plot this graph to give you the curve.

Then where the curve crosses the line y=0 (so the x axis) is the values of x.

Is this clearer? :D

Answered Sun 17th June, 2012 @ 14:14 by Dilly
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yes- thank you:) I just got a bit confused because it says =0 , instead of y= ...:)

Answered Sun 17th June, 2012 @ 14:23 by AmyPond
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Oohh. Well you could think of it like this...

If y=x^3+6x^2-19x+84 and y=0...then x^3+6x^2-19x+84 must = 0 because they both equal y! :D

Answered Sun 17th June, 2012 @ 14:36 by Dilly
Edited by Dilly on Sun 17th June, 2012 @ 14:36
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ok:) thank you!

Answered Sun 17th June, 2012 @ 14:39 by AmyPond
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No problem! :D

Answered Sun 17th June, 2012 @ 14:40 by Dilly