Difference between revisions of "Definite integral"
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==Definite Integrals== | ==Definite Integrals== | ||
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A definite integral is the area under the curve between two points on the function. In the picture below, the yellow area is "positive" and the blue area is "negative". The integral is evaluated by adding the positive area together and subtracting the negative area. | A definite integral is the area under the curve between two points on the function. In the picture below, the yellow area is "positive" and the blue area is "negative". The integral is evaluated by adding the positive area together and subtracting the negative area. | ||
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If the function f(x) is real rather than complex, then the definite integral is also known as a Riemann integral. | If the function f(x) is real rather than complex, then the definite integral is also known as a Riemann integral. | ||
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== Solving Definite Integrals == | == Solving Definite Integrals == | ||
Solving a definite integral usually has two main steps: [[integration]] and [[subtraction]]. | Solving a definite integral usually has two main steps: [[integration]] and [[subtraction]]. | ||
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== Example 1== | == Example 1== | ||
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: <math>F(5) = {1 \over 3}5^3 = {125 \over 3}</math> | : <math>F(5) = {1 \over 3}5^3 = {125 \over 3}</math> | ||
: <math>F(-3) = {1 \over 3}(-3)^3 = {-27 \over 3}</math> | : <math>F(-3) = {1 \over 3}(-3)^3 = {-27 \over 3}</math> | ||
| + | <br />And subtract: | ||
: <math>{125 \over 3} - {-27 \over 3} = {125 \over 3} + {27 \over 3} </math> | : <math>{125 \over 3} - {-27 \over 3} = {125 \over 3} + {27 \over 3} </math> | ||
: <math>= {152 \over 3}</math> | : <math>= {152 \over 3}</math> | ||
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: <math>\int_4^{12}\frac{3x+11}{x^2-x-6}dx </math> | : <math>\int_4^{12}\frac{3x+11}{x^2-x-6}dx </math> | ||
=== Integration === | === Integration === | ||
| − | <br />See the [[Partial fractions in integration]] page for how to integrate. | + | <br />See the [[Partial fractions in integration]] page for how to integrate the above expression (it is the example). |
<br />As shown on the page mentioned above: | <br />As shown on the page mentioned above: | ||
: <math>\int\frac{3x+11}{x^2-x-6}dx=4ln|x-3|-ln|x+2|+c</math> | : <math>\int\frac{3x+11}{x^2-x-6}dx=4ln|x-3|-ln|x+2|+c</math> | ||
Revision as of 21:23, December 30, 2008
A definite integral is an integral with upper and lower limits.
Definite Integrals
A definite integral is the area under the curve between two points on the function. In the picture below, the yellow area is "positive" and the blue area is "negative". The integral is evaluated by adding the positive area together and subtracting the negative area.
If the function f(x) is real rather than complex, then the definite integral is also known as a Riemann integral.
Solving Definite Integrals
Solving a definite integral usually has two main steps: integration and subtraction.
Example 1
This is a very simple definite integral:
Integration
Using indefinite integration, it can be shown that:
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \int x^2dx = {1 \over 3} x^3 = F(x)}
Note that Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle F(x)}
is the indefinite integral of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f(x)}
.
Subtraction
Now, plug Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 5}
and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle -3}
into the new expression and subtract, as shown by the Fundamental Theorem of Calculus.
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle F(5) = {1 \over 3}5^3 = {125 \over 3}}
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle F(-3) = {1 \over 3}(-3)^3 = {-27 \over 3}}
And subtract:
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle {125 \over 3} - {-27 \over 3} = {125 \over 3} + {27 \over 3} }
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle = {152 \over 3}}
Example 2
This is a more complex definite integral that requires partial fractions to solve:
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \int_4^{12}\frac{3x+11}{x^2-x-6}dx }
Integration
See the Partial fractions in integration page for how to integrate the above expression (it is the example).
As shown on the page mentioned above:
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \int\frac{3x+11}{x^2-x-6}dx=4ln|x-3|-ln|x+2|+c}
Subtraction
This means that we can now subtract:
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \left [ 4ln|12-3|-ln|12+2|+c \right ] - \left [ 4ln|4-3|-ln|4+2|+c \right ]}
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle = \left [ 4ln|9|-ln|14|+c \right ] - \left [ 4ln|1|-ln|6|+c \right ]}
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle = 4ln|9|-ln|14| - 4ln|1| + ln|6|}
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle = 4ln|9|-ln|14| + ln|6|}
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \approx 6.02941...}
Note the following:
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle ln(1) = 0}
- The Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle c} on each side cancels out because we have Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle c-c}
- Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle ln|6|} becomes positive because it was Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle -(-ln|6|)}