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Unit 7

7.4 Reasoning Using Slope Fields

1 min readโ€ขjune 8, 2020

Jacob Jeffries


Continued from 7.3 Sketching Slope Fields.

The actual solution (which can actually be manipulated to be separable*) to the differential equation in Eq. 39 is the following:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreenshot%20(936).png?alt=media&token=76c65c2e-f550-407b-94aa-0619221dcebf

By going to https://www.desmos.com/calculator/fjli4efhcj and clicking the play button on equation 18, you can see that this curve does indeed fit the given slope field for any constant C. Clicking the play button will show different curves for different values of C.

The most intuitive way to think of a slope field is to picture a fluid flowing and then placing an object on the fluid that will trace out a path. This path is approximate toย the solution to the curve that represents the differential equation. ๐Ÿ˜€

Resources:

Review

Fill in the table below for different values of yโ€™ at different coordinate points. Use a calculator to find the values to two decimal places. Create a slope field and then solve the differential equation and confirm that your slope field matches the solution to the differential equation. โœ

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreenshot%20(939).png?alt=media&token=b3092ad5-9705-4de3-87ad-a255333ad2bf

Answer

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreenshot%20(942).png?alt=media&token=c2a610f4-7a97-48cb-893f-e725166dffac

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreenshot%20(943).png?alt=media&token=5e664e30-690c-4389-88f7-f6a13da5032a

Varying values of C plotted over the slope field are shown here:

https://www.desmos.com/calculator/ab0swk21ao


Footnotes

*One can make this separable by doing a substitution:

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2FScreenshot%20(937).png?alt=media&token=876fb78c-7fe6-44a7-af2b-3f1ac0e4074f

From here, one can solve the latter differential equation (which will give a solution that is only a function of x) and substitute this into u = x + y, which will give the aforementioned solution to the original differential equation.

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