Definite Integral
- The definite integral is a fundamental concept in calculus.
- It represents the area under a curve between two specified limits.
- It is denoted by ∫ from a to b f(x) dx.
- The integral of a function represents the signed area enclosed between the curve and the x-axis.
- The definite integral can be used to calculate displacement, velocity, area, and many other quantities in mathematics and physics.
Example:
Let’s find the definite integral ∫ from 0 to 2 of x^2 dx.
Solution:
Using the power rule of integration, we have:
∫x^2 dx = (x^3)/3
Substituting the limits, we get:
∫ from 0 to 2 of x^2 dx = [(2^3)/3] - [(0^3)/3] = 8/3 - 0 = 8/3
Therefore, the definite integral of x^2 from 0 to 2 is 8/3.
Equation:
The definite integral is given by the equation:
∫ from a to b f(x) dx = F(b) - F(a)
where F(x) is the antiderivative of f(x) with respect to x.
Rules of Definite Integrals
- Linearity Property: ∫[af(x) + bg(x)] dx = a*∫f(x) dx + b*∫g(x) dx
- Sum/Difference Property: ∫[f(x) ± g(x)] dx = ∫f(x) dx ± ∫g(x) dx
- Constant Multiple Property: ∫kf(x) dx = k∫f(x) dx
- Change of Limits: ∫ from a to b f(x) dx = -∫ from b to a f(x) dx
Example:
Let’s evaluate the definite integral ∫ from 0 to π of (sin x + cos x) dx.
Solution:
Using the sum property of definite integrals, we can split the integral into two parts:
∫ from 0 to π of sin x dx + ∫ from 0 to π of cos x dx
Using the integral properties and evaluating separately, we find:
[-cos x] from 0 to π + [sin x] from 0 to π
= [-cos π + cos 0] + [sin π - sin 0]
= [(-1) + 1] + [0 - 0]
= 0 + 0
= 0
Therefore, the definite integral of (sin x + cos x) from 0 to π is 0.
Properties of Definite Integrals
- Change of Variable: If u = g(x), then: ∫ from a to b f(g(x)) g’(x) dx = ∫ from g(a) to g(b) f(u) du
- Symmetry Property: ∫ from -a to a f(x) dx = 0 (if f(x) is an odd function)
- Even Function Property: ∫ from -a to a f(x) dx = 2∫ from 0 to a f(x) dx (if f(x) is an even function)
- Additive Property: ∫ from a to b f(x) dx + ∫ from b to c f(x) dx = ∫ from a to c f(x) dx
Example:
Let’s evaluate the definite integral ∫ from -2 to 2 x^3 dx.
Solution:
Using the symmetry property of definite integrals, we can rewrite the integral as:
2∫ from 0 to 2 x^3 dx
Using the power rule of integration, we have:
2*(x^4)/4 evaluated from 0 to 2
= (2^4)/4 - (0^4)/4
= 16/4 - 0/4
= 4
Therefore, the definite integral of x^3 from -2 to 2 is 4.
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Slide 11: Definite Integral - Example of integral
- Let’s solve an example to find the definite integral.
- Example: Find the value of ∫ from 1 to 3 (2x + 3) dx.
- Solution: Integrating term by term, we get ∫(2x + 3) dx = x^2 + 3x.
- Substituting the limits, we have [x^2 + 3x] evaluated from 1 to 3.
- Evaluating at x = 3: (3^2 + 3*3).
- Evaluating at x = 1: (1^2 + 3*1).
- Simplifying, we find the value of the definite integral.
Slide 12: Definite Integral - Area Interpretation
- The definite integral can be interpreted as the area under a curve.
- It represents the accumulation of infinitesimally small areas.
- The integral of a positive function gives the area above the x-axis.
- The integral of a negative function gives the area below the x-axis.
- The definite integral can also calculate the area between two curves.
Slide 13: Definite Integral - Area between Curves
- To find the area between two curves, we subtract the lower function from the upper function.
- Example: Find the area between the curves y = x^2 and y = x.
- Solution: Set the equations equal to each other and solve for the x-values.
- The intersection points are x = 0 and x = 1.
- We integrate the upper function minus the lower function: A = ∫ (x - x^2) dx.
- Evaluating the integral with limits 0 to 1, we find the area.
Slide 14: Properties of Definite Integrals - Change of Variable
- The change of variable property allows us to use a new variable in integration.
- Example: Let u = 2x + 1. We want to find ∫(2x + 1) dx.
- Substituting the new variable, we have: ∫u du/2.
- Integrating, we get u^2/4 + C.
- Substituting back the original variable, we find the result in terms of x.
Slide 15: Properties of Definite Integrals - Symmetry Property
- The symmetry property states that for an odd function, the definite integral from -a to a is zero.
- An odd function satisfies f(-x) = -f(x).
- Example: Find ∫ from -2 to 2 x^3 dx.
- Since x^3 is an odd function, the integral evaluates to zero.
Slide 16: Properties of Definite Integrals - Even Function Property
- The even function property states that for an even function, the definite integral from -a to a is twice the integral from 0 to a.
- An even function satisfies f(-x) = f(x).
- Example: Find ∫ from -3 to 3 x^2 dx.
- Since x^2 is an even function, we can rewrite the integral as 2 times the integral from 0 to 3.
- Evaluating both integrals, we find the result.
Slide 17: Properties of Definite Integrals - Additive Property
- The additive property states that the sum of definite integrals from a to b and from b to c is equal to the integral from a to c.
- Example: Given ∫ from 1 to 3 x dx and ∫ from 3 to 4 x dx, find ∫ from 1 to 4 x dx.
- Using the additive property, we add the two integrals to find the final result.
Slide 18: Definite Integral - Applications in Calculus
- The definite integral has various applications in calculus.
- It can be used to calculate displacement, velocity, acceleration, and other quantities.
- The area under a velocity-time graph gives the displacement.
- The area under an acceleration-time graph gives the change in velocity.
- The definite integral is also used in finding the volume of solids of revolution.
Slide 19: Definite Integral - Applications in Physics
- In physics, the definite integral is used to calculate work done.
- Work is defined as the force applied over a displacement.
- The area under a force-displacement graph gives the work done.
- The definite integral is also used in finding the center of mass and moment of inertia.
Slide 20: Definite Integral - Summary
- The definite integral represents the accumulation of infinitesimal areas.
- It can be interpreted both as an area under a curve and as a method for calculation.
- The properties of definite integrals allow for simplification and manipulation.
- It has applications in calculus, physics, and various other fields.
- Understanding the concept and techniques of definite integrals is important in advanced mathematics.
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