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How can one limit the range of a parabola on a graphical display? (eg TI-84 Plus Silver...

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jasminehems | Student, Undergraduate | Honors

Posted May 7, 2013 at 2:02 AM via web

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How can one limit the range of a parabola on a graphical display? (eg TI-84 Plus Silver Edition)



Here, how would one limit the range of the parabola to the points where it cuts the red line y=1?

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oldnick | Valedictorian

Posted May 7, 2013 at 2:28 AM (Answer #1)

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Whatever your calculator is , you have to calcualte the value of x for parabola intersecates red line.

I.e your parabola is `y=8-2/9x^2`

that has zero for `x=+-6`

To provivde end of graphic no over the interestion with red line `y=1`  you have to find solution of equation:

`8-2/9x^2=1`  `rArr`  `-2/9x^2=-7`   `rArr x= +-3/2 sqrt(14)`

`x=+- 5.6124860801609120783756230984748`

So if you set this range claculator does not draw graphic once intersects straight line.  As in figure bellow:

 

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mathsworkmusic | (Level 3) Associate Educator

Posted May 10, 2013 at 10:24 AM (Answer #2)

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You need to limit the x range of the parabola to the points where the parabola crosses the red line.

Since the parabola intersects the x axis at +/- 6, and since the coefficient in x^2 is negative (the parabola is inverted) the formula for the parabola is

` ` `y = -a(x-6)(x+6)`

`= -a(x^2 - 36) = 36a - ax^2`

Also, since the intercept is at `y=8`, this implies that `a = 8/36 = 2/9` . The formula is then

`y = 8 - 2/9x^2`

To find where the parabola crosses the red line, solve

`8 - 2/9x^2 = 1`

ie,  `2/9x^2 = 7`

This implies that the parabola crosses the line when  `x = pm sqrt(63/2) = pm 5.6125`

Limit the x range of the parabola to between -5.6125 and +5.6125 in the RANGE field.

 

 

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