1.4 Antiderivative, Definite integral
*****************************Antiderivative*****************************
Assuming that you know how to take a derivative of a function, you are now ready to take the inverse of a derivative.
Theorem 1.4.1
Let f be any function.
Then it's Antiderivative (in general form) is F(x) + C, where C is an arbitrary constant.
Example 1.4.1
Find the general antiderivative of the function.
f(x) = x2 + x + 5
Solution: The general antiderivative of f(x) is
F(x) = (1/3)x3 + (1/2)x2 + 5x + C.
Example 1.4.2
Find the antiderivative of the trigonometric function.
f(x) = cos(x) + sin(x)
Solution: The derivative of cos(x) is -sin(x) and sin(x) is cos(x).
So the antiderivative of f(x) is the opposite.
Therefore, F(x) = sin(x) + [-cos(x)] = sin(x) - cos(x).
Taking antiderivative of a polynomial function is like using Power Rule, but instead, you add one to the exponent and divide by the new exponent.
For the trigonometric functions, they alternate with negative sign.
Problem 1.4.1
Take the antiderivative of f(x) = 1/x2.
Problem 1.4.2
Find f(x) if f''(x) = (x)1/2
****************************Definite Integral***************************
Now you are ready to do the integral. For definite integral, there exists both upper limit* and lower limit* as real numbers.
Definition 1.4.1
The definite integral of f form a to b is ∫baf(x) dx,
*where upper limit is a and lower limit is b.
Example 1.4.3
Evaluate the integral.
∫01 x dx
Solution: First, antiderivative of x is (1/2)x2.
So, ∫01 x dx = (1/2)x2 l01 = (1/2)(1)2 - (1/2)(0) = 1/2.
Example 1.4.4
Evaluate the integral.
∫01 cos(x) dx
Solution: ∫01 cos(x) dx = sin(x) l01 = sin(1) - sin(0) = sin(1).
Problem 1.4.3
Evaluate ∫-11 2x dx.
Problem 1.4.4
Prove that ∫ba x dx = (b2 - a2)/2.
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