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6.2 Integration by Substitution & Separable Differential Equations

6.2 Integration by Substitution & Separable Differential Equations. Greg Kelly Hanford High School Richland, Washington. M.L.King Jr. Birthplace, Atlanta, GA. Photo by Vickie Kelly, 2002.

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6.2 Integration by Substitution & Separable Differential Equations

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  1. 6.2 Integration by Substitution & Separable Differential Equations Greg Kelly Hanford High School Richland, Washington M.L.King Jr. Birthplace, Atlanta, GA Photo by Vickie Kelly, 2002

  2. The chain rule allows us to differentiate a wide variety of functions, but we are able to find antiderivatives for only a limited range of functions. We can sometimes use substitution to rewrite functions in a form that we can integrate.

  3. Example 1: The variable of integration must match the variable in the expression. Don’t forget to substitute the value for u back into the problem!

  4. One of the clues that we look for is if we can find a function and its derivative in the integrand. The derivative of is . Note that this only worked because of the 2x in the original. Many integrals can not be done by substitution. Example: (Exploration 1 in the book)

  5. Example 2: Solve for dx.

  6. Example 3:

  7. We solve for because we can find it in the integrand. Example: (Not in book)

  8. Example 7:

  9. The technique is a little different for definite integrals. new limit new limit Example 8: We can find new limits, and then we don’t have to substitute back. We could have substituted back and used the original limits.

  10. Leave the limits out until you substitute back. This is usually more work than finding new limits Example 8: Using the original limits: Wrong! The limits don’t match!

  11. Example: (Exploration 2 in the book) Don’t forget to use the new limits.

  12. Separable Differential Equations A separable differential equation can be expressed as the product of a function of x and a function of y. Example: Multiply both sides by dx and divide both sides by y2 to separate the variables. (Assume y2 is never zero.)

  13. Separable Differential Equations A separable differential equation can be expressed as the product of a function of x and a function of y. Example: Combined constants of integration

  14. Example 9: Separable differential equation Combined constants of integration

  15. Example 9: We now have y as an implicit function of x. We can find y as an explicit function of x by taking the tangent of both sides. Notice that we can not factor out the constant C, because the distributive property does not work with tangent.

  16. In another generation or so, we might be able to use the calculator to find all integrals. Until then, remember that half the AP exam and half the nation’s college professors do not allow calculators. You must practice finding integrals by hand until you are good at it! p

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