# Values of M so 2 curves don't intersect.

• trollcast
In summary, the range of values for m in which the curves f(x) and g(x) do not intersect is between -6 and 6. The method of setting the equations equal to each other and solving for x is more efficient than finding the tangents and solving for m.
trollcast
Gold Member

## Homework Statement

2 curves f(x) and g(x) don't intersect, find the range of values of m can be.

## Homework Equations

$$f(x)=3x^2 - 2$$
$$g(x) = mx-5$$

## The Attempt at a Solution

Could I work out the 2 values for m that mean g(x) is a tangent to f(x) at some point and then the range will be between them?

PS. I've already solved it via another approach but I was just wondering if this would work as it was my first initial idea on solving it.

Last edited:
You're on the right track, the limiting values for m making the g(x) line a tangent to f(x) is correct.

have you plotted 3x^2 - 2 ? that would give you a hint of what the values of m are.

for g(x) you know one one point for certain: the y-intercept

jedishrfu said:
You're on the right track, the limiting values for m making the g(x) line a tangent to f(x) is correct.

have you plotted 3x^2 - 2 ? that would give you a hint of what the values of m are.

for g(x) you know one one point for certain: the y-intercept

I tried it again and I must have made a mistake when I tried this the first time as I definitely didn't get this.

For g(x) to be a tangent of f(x) their gradients must be the same:
$$f'(x)=6x \\ g(x)=6x(x)-5 \\ g(x)=6x^2-5$$

And they also must intersect each other:$$f(x)=g(x)\\ 3x^2-2=6x^2-5\\ 3x^2-3=0\\ x^2-1=0\\ x^2=1\\ x=\pm1$$
Then just work out the values of m for the values of x:$$m=f'(x)\\ m=6x\\ m=6(\pm1)\\ m=\pm6\\ -6<m<6$$

and you can check this by plotting the two lines and the original f(x).

I'm not sure you should focus on the slope. The two graphs will intersect when $3x^2- 2= mx- 5$ which is the same as $3x^2- mx+ 3= 0$.

The "discriminant" of that is $$(-m)^2- 4(3)(3)= m^2- 36= 0$$. That is a parabola that is negative for x between -6 and 6. Since a quadratic equation has no real solutions if the discriminant is negative, the graphs do not intersect for m between -6 and 6.

HallsofIvy said:
I'm not sure you should focus on the slope. The two graphs will intersect when $3x^2- 2= mx- 5$ which is the same as $3x^2- mx+ 3= 0$.

The "discriminant" of that is $$(-m)^2- 4(3)(3)= m^2- 36= 0$$. That is a parabola that is negative for x between -6 and 6. Since a quadratic equation has no real solutions if the discriminant is negative, the graphs do not intersect for m between -6 and 6.

Thanks,

That was the method I used in class to solve it after I tried working out the tangents but made a mistake copying something.

Is there any advantage to that method, ie. would the tangents not work for some similar problems?

## 1. What does the "M" value represent in a curve?

The "M" value in a curve represents the slope or gradient of the line. It is a measure of how steep the curve is.

## 2. Why is it important for the M values to be different in two curves?

If the M values are the same in two curves, they will intersect at some point. This can lead to confusion and inaccurate data analysis. Having different M values ensures that the curves do not intersect and can be easily distinguished from each other.

## 3. How do you determine the M values in a curve?

The M value can be determined by calculating the change in the Y-axis values divided by the change in the X-axis values between two points on the curve. This is also known as the rise over run.

## 4. Can the M values of two curves ever be equal?

Yes, it is possible for the M values of two curves to be equal. This usually occurs when the two curves are parallel to each other.

## 5. What happens if the M values of two curves are equal?

If the M values of two curves are equal, it means that the curves are parallel and will never intersect. This can be useful in data analysis to show a constant relationship between two variables.

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