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# GCSE maths quadratic graph question watch

1. The question is " the diagram shows a sketch of the graph y=x^2+ax+b
the graph crosses the x-axis at (2,0) and (4,0) work out the value of b"
Can anyone help?
2. Sorry, nope.
3. firstly let x = 2 and y = 0

y = x2 + ax + b

then repeat with x = 4 and y = 0

then solve the two equations simultaneously
4. (Original post by ameehannah)
...

No idea if this is correct, but when your quadratic graph crosses the axis, , so set your equation equal to 0. You have two roots, so you sub in the terms from each root, to get 2 expressions, and you then solve them simultaneously.

I just checked it, and it's correct. Try this method. Looking over it, not sure if my method is the most elegant; think there's a more obvious way of doing it.

Well, if this function has roots and , then this means it can be factorised as (by the factor theorem, although you won't know what this is, generally if a value is a root, then is a factor).

Remember, what are the roots? (I don't want to just give you the answer.)

You could then expand that, and look at the values you have for .
5. (Original post by ameehannah)
The question is " the diagram shows a sketch of the graph y=x^2+ax+b
the graph crosses the x-axis at (2,0) and (4,0) work out the value of b"
Can anyone help?
Alternatively, as you are given the roots of the quadratic, you know it can be written as . So you can expand this to find the value of b.
6. (Original post by brittanna)
Alternatively, as you are given the roots of the quadratic, you know it can be written as . So you can expand this to find the value of b.
thanks
7. (Original post by ameehannah)
thanks
No problem. But remember that you can only do this as they are roots of the equation (i.e. when y=0). If you were told that it passes through the points (2, 1) and (4, 3) for example, then you would have to use the method the previous posters have said.
8. (Original post by brittanna)
No problem. But remember that you can only do this as they are roots of the equation (i.e. when y=0). If you were told that it passes through the points (2, 1) and (4, 3) for example, then you would have to use the method the previous posters have said.
Thanks very much, I'll remember that

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