Applied Calculus: UNIT III: Integral Calculus

Comparison Test for Improper Integrals

Explanation, Formula, Equation, Example and Solved Problems - Integral Calculus: Comparison Test for Improper Integrals

COMPARISON TEST FOR IMPROPER INTEGRALS

 

Suppose that f and g are continuous functions with f(x) ≥ g(x) ≥ 0 for x≥ a.

If a f(x) dx is convergent, then ag(x) dx is convergent.

If a g(x) dx is convergent, then af(x) dx is convergent.

 

Example 101. Does the integral  is convergent?

Solution:

We know that 1/x2 > 1/(ex+x2) > 0.

Here 1 dx/x2 is convergent by the p‒test, since p = 2 > 1.

Therefore by comparison test,  is convergent

 

Example 102. Determine if the following integral is convergent or divergent.


Solution: We know that


So 3e‒xdx is convergent. Therefore, by the Comparison test


is also convergent.

 

Example 103. Does the integral 1dx/xex converge?

Solution:

We have 1/ex > 1/xex > 0.


 1dx/ex convergent

Therefore by comparison test, the integral 1dx/xe‒x converges

 

Example 104. Does the integral 1dx/√[1+x3] converges

Solution:

We have 1/√x3 > 1/(√[1+x3]) > 0.

Here 1 dx/√x3 is convergent by the p‒test, since p = 3/2 > 1.

Therefor by comparison test, the integral 1dx/√[1+x3] converges.

 

Example 105. Does the integral  converge?

Solution:

 is divergent by the p‒test, since p = 2/5 ≤ 1.

Therefore by comparison test, the integral  diverges.

 

Example 106. Does the integral 2(2+ sinx)/(x‒1) converge?

Solution: We have (2+ sin x)/(x‒1) > 1/x > 0

Note that 2 dx/x is divergent by the p‒test, since p = 1 ≤ 1.

Therefore by comparison test, the integral 2(2+ sin x)/(x‒1) diverges.

 

Example 107. Determine if the following integral is convergent or divergent.


Solution:

We now know that we need to find a function that is larger than

 cos2(x) / x2

We can use the fact that 0 ≤ cos2(x) ≤ 1 to make the numerator larger (i.e. we'll replace the cosine with something we know to be larger, namely 1). So,


converges since p = 2 > 1converges and so by the Comparison Test we know that


must also converge.

 

 

EXERCISE

 

13. Check whether the following improper integral converges or diverges, if converges evaluate it.


 

Applied Calculus: UNIT III: Integral Calculus : Tag: Applied Calculus : - Comparison Test for Improper Integrals


Applied Calculus: UNIT III: Integral Calculus



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