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4.07 Further definite integrals

Lesson

Using our knowledge of integration of exponential and trigonometric functions we can evaluate definite integrals and find areas under the curves of such functions.

Keep the following rules in mind as we look at some examples.

Function $f\left(x\right)$f(x) Integral $\int f\left(x\right)dx$f(x)dx
$e^{ax+b}$eax+b $\frac{1}{a}e^{ax+b}+C$1aeax+b+C
$\cos\left(ax+b\right)$cos(ax+b) $\frac{1}{a}\sin\left(ax+b\right)+C$1asin(ax+b)+C
$\sin\left(ax+b\right)$sin(ax+b) $-\frac{1}{a}\cos\left(ax+b\right)+C$1acos(ax+b)+C

 

Graph symmetry

We can make use of the symmetry properties of functions, including the period of trigonometric functions and whether a function is odd or even to simplify and more easily evaluate integrals and areas bounded by curves.

Odd function Even function

Example:

Example:

For odd functions: $f\left(-x\right)=-f\left(x\right)$f(x)=f(x)

and $\int_{-a}^a\ f(x)\ dx=0$aa f(x) dx=0

For even functions:$f\left(-x\right)=f\left(x\right)$f(x)=f(x)

and $\int_{-a}^a\ f(x)\ dx=2\int_0^a\ f(x)\ dx$aa f(x) dx=2a0 f(x) dx

 

Periodic or other symmetries

Example:

For the above trigonometric curve we could use the following symmetry properties of this function:

$\int_a^c\ f(x)\ dx=0$ca f(x) dx=0 and the shaded area is given by $A=2\int_a^b\ f(x)\ dx$A=2ba f(x) dx

 

Worked examples

Example 1

Find the exact value of

Think: Use the fact that $\int e^{ax+b}\ dx=\frac{1}{a}e^{ax+b}+C$eax+b dx=1aeax+b+C and evaluate the integral at the endpoints.

Do:

Example 2

Find the area bounded by the curve $y=\sin\left(2x\right)$y=sin(2x), the coordinate axes, and the line $x=\frac{3\pi}{2}$x=3π2.

Think: Since we require the area and not simply the integral we will sketch the function and observe which regions of the graph lie below the $x$x-axis. We can then add the integral for regions which lie above the $x$x-axis to the absolute value of those below or use symmetry properties of the graph.

Do:

From the graph we can see that to find the area we could calculate:

Or using the symmetry of the graph we can simplify the calculation to:

Evaluating this we obtain:

Thus the area bounded by the curve $y=\sin\left(2x\right)$y=sin(2x), the coordinate axes, and the line $x=\frac{3\pi}{2}$x=3π2 is $3$3 units2.

 

Practice questions

Question 1

Find the exact value of $\int_0^2e^{\frac{3}{2}x}dx$20e32xdx.

Question 2

Calculate the area enclosed between the curve $y=e^x$y=ex, the coordinate axes, and the line $x=2$x=2.

Loading Graph...

Question 3

Consider the following.

  1. Given that $y=e^{3x}\left(x-\frac{1}{3}\right)$y=e3x(x13), determine $y'$y.

    You may use the substitutions $u=e^{3x}$u=e3x and $v=\left(x-\frac{1}{3}\right)$v=(x13) in your working.

  2. Hence find the exact value of $\int_6^9xe^{3x}dx$96xe3xdx.

Question 4

Evaluate $\int_{-\frac{\pi}{6}}^{\frac{\pi}{6}}\left(4\cos x+\cos4x\right)dx$π6π6(4cosx+cos4x)dx.

Question 5

Find the exact area of the shaded regions bounded by the curve $y=3\cos x$y=3cosx.

Loading Graph...

 

Using the calculator

Using technology we can evaluate definite integrals and find areas under curves, in fact there are many functions where algebraic integration is not possible. Using approximation methods, limits and technology are necessary tools to evaluate integrals in such cases.

Use your calculator and attempt the practice questions below.

Reminder

Ensure your calculator is in radians mode before sketching or calculating integrals for trigonometric functions.

Practice questions

Question 6

Use technology to evaluate $\int_{-1}^4\frac{e^x}{\sqrt{x+4}}dx$41exx+4dx.

Round your answer to two decimal places.

Question 7

Use technology to calculate the area bounded by $y=e^{2x}\cos3x$y=e2xcos3x, the $x$x-axis, and the lines $x=-\frac{\pi}{2}$x=π2 and $x=\frac{\pi}{6}$x=π6.

Round your answer to two decimal places.

Outcomes

3.2.12

interpret the definite integral ∫ {from a to b} f(x)dx as area under the curve y=f(x) if f(x)>0

3.2.13

interpret ∫ {from a to b} f(x)dx as a sum of signed areas

3.2.14

apply the additivity and linearity of definite integrals

3.2.17

develop the formula ∫ {from a to b} f(x)dx= F(b)−F(a) and use it to calculate definite integrals

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