inexact calculation

pl shared this question 7 months ago

I want to find meeting point of

y=-0.01x^2 +0.69x +1.5


y=0.5x -30

this should give x=66.42 and y=3.21

Goegebra shows for Point F x=66.54 and for y=3.27 !

This is not a big difference but it is a difference that shouldn't be!

Comments (3)


Maybe GoeGebra makes a mistake, but GeoGebra solves it right in three different ways:

1. Assign the equations to functions f and g

2. Draw them as two curves y = ...

3. Use the intersection tool

The results for the points B, D and E are three times the ones you mentions as the exact results, so I presume the program does it all right. Are you sure that the numbers shown in the algebra and the sliders aren't just rounded ones and not the ones you really use in the construction which produces a difference for the intersection point?




Thanks Chris for replying so fast -

yes this way I also come to the same results -

but when I work with rulers, though they show the same (!) values (and I even set the right stepping size) for the parameters - the result with the intersection tool is different ! - shouldn't be to my oppinion...



If you set the rounding of your file to more than 2, you can see that the value of b doesn't equal 0.69 but 0.692. This must have happened while manipulating and dragging the slider at a certain moment. And ideed if you zoom in very much, you can see that when using the slider values you get a slightly different graph (and intersection point) then when typing the exact numbers, due to this little difference. So hint: always check roundings! And it's also why adding ggb files is more useful to solve things than screen captures.

Files: b.PNG
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