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# How to sketch quadratic? watch

1. For example: y = (5 - 8x - x^2) (x + 4)

I know how to find all the points etc but I I just don't know how to draw sketch the graph... Is there any way to work out were the graph starts from and how it looks?
2. That isn't a quadratic, it is a cubic. You are expected to know the general shape of a negative cubic.
3. (Original post by IShouldBeRevising_)
For example: y = (5 - 8x - x^2) (x + 4)

I know how to find all the points etc but I I just don't know how to draw sketch the graph... Is there any way to work out were the graph starts from and how it looks?
Er, that's not a quadratic - it's a cubic

Assuming that's the function you want to graph, it's not too difficult to sketch a cubic in general. Remember that when x is very big the term in x^3 is much bigger than anything else, so it dominates.

If you have a function with a positive number in front of the x^3 e.g. y = 4x^3 + something, then when x is very big and negative, y will also be big and negative; when x is big and positive, y is big and positive. So the graph starts at the "bottom left" and ends up "at the top right".

If you have a function with a negative number if front of the x^3 e.g. y = -2x^3 + something, then the behaviour is reversed - large negative x gives large positive y, and vice versa. So you start up in the "top left" and end up at the "bottom right"

So work out what's in front of your x^3 and use that to work out the large scale behaviour of the function. Next look for obvious zeros of the function and/or turning points.
4. cubic graphs do not "start from" anywhere...

you should find the x intercepts (roots) and the y intercept. then draw the curve with two turning points.

job done
5. (Original post by the bear)
cubic graphs do not "start from" anywhere...

you should find the x intercepts (roots) and the y intercept. then draw the curve with two turning points.

job done
everything "starts from" somewhere...unless you have an infinite sheet of graph paper
6. (Original post by Mr M)
That isn't a quadratic, it is a cubic. You are expected to know the general shape of a negative cubic.

(Original post by davros)
Er, that's not a quadratic - it's a cubic

Assuming that's the function you want to graph, it's not too difficult to sketch a cubic in general. Remember that when x is very big the term in x^3 is much bigger than anything else, so it dominates.

If you have a function with a positive number in front of the x^3 e.g. y = 4x^3 + something, then when x is very big and negative, y will also be big and negative; when x is big and positive, y is big and positive. So the graph starts at the "bottom left" and ends up "at the top right".

If you have a function with a negative number if front of the x^3 e.g. y = -2x^3 + something, then the behaviour is reversed - large negative x gives large positive y, and vice versa. So you start up in the "top left" and end up at the "bottom right"

So work out what's in front of your x^3 and use that to work out the large scale behaviour of the function. Next look for obvious zeros of the function and/or turning points.

(Original post by the bear)
cubic graphs do not "start from" anywhere...

you should find the x intercepts (roots) and the y intercept. then draw the curve with two turning points.

job done
Yeah I meant a cubic... not quadratic. Yeah Davros thanks for that info.
7. (Original post by davros)
everything "starts from" somewhere...unless you have an infinite sheet of graph paper
where do circles start from ? hmmm ? hmmm ?

mwahahahahahah

8. (Original post by the bear)
where do circles start from ? hmmm ? hmmm ?

mwahahahahahah

oh no, circles are undrawable...I don't know where to start from

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