# Notes-Antiderivatives

```Antiderivatives
Before: Given f (x), find f ′(x).
Now: f (x), find F(x) such that F ′(x) = f (x).
Definition: A function F is called an antiderivative of f on an interval I if
F ′(x) = f (x) for all x in I.
Ex. Because (sin x)′ = cos x, therefore F(x) = sin x is an antiderivative of
f (x) = cos x.
Recall that, as a consequence of the Mean Value Theorem, all functions with
the same derivative differ from each other by a constant. Therefore,
continue the example above, functions of the form F(x) = sin x + C, where C
is any constant, is the set of all antiderivatives of f (x) = cos x.
Theorem: If F is an antiderivative of f on an interval I, then the most
general antiderivative of f on I is
F(x) + C
where C is an arbitrary constant.
Comment: This theorem tells us that, in order to find all
antiderivatives of a given function f , all we need to do is to find ONE
function F whose derivative is F ′ = f . The rest of the antiderivatives
of f can then be found by adding constants to F. The family of all
antiderivatives of a function f , F(x) + C, is called the indefinite
integral of f .
The process of finding antiderivatives of a given function is called
integration. It is the reverse process of differentiation.
Notation: Integration and Indefinite Integral
The fact that the set of functions F(x) + C represents all
antiderivatives of f (x) is denoted by:
∫ f ( x)dx = F ( x) + C
where the symbol ∫ is called the integral sign, f (x) is the integrand, C is the
constant of integration, and dx denotes the independent variable we are
integrating with respect to.
Ex. Power rule of integration
d  1 n +1 
1 n+1
x  = x n , the function F ( x) =
x is an

dx  n + 1
n +1

antiderivative of f (x) = xn, for n ≠ −1. Therefore, the set of all
antiderivatives of xn is:
Because
n
∫ x dx =
1 n+ 1
x + C , for all numbers n, n ≠ −1.
n +1
Math 141 notes: Later you’ll learn the “missing” case of the above
power rule, the case where n = −1. It is
1
−1
x
dx
=
∫
∫ x dx = ln x + C
Ex.
∫
Ex.
∫  x
=
5

x dx = ∫ x 1 / 5 dx =
2
+
1 3
x − x −1
3
1
−2
x2
4
− x3/ 2
3
1 6/5
5
x + C = x6/5 + C
6/5
6
1
1 −1
1 3/ 2

x dx = ∫ x 2 + x − 2 − 2 x 1 / 2 dx = x 3 +
x −2
x +C
3
3/ 2
−1

(
+C
)
Some other basic integration formulas
sum/difference rule:
∫ [ f ( x) &plusmn; g ( x)]dx = ∫ f ( x)dx &plusmn; ∫ g ( x)dx
constant-multiple rule:
∫ c f ( x)dx = c ∫ f ( x)dx
∫ cos x dx = sin x + C
∫ sin x dx = −cos x + C
∫ sec2 x dx = tan x + C
∫ sec x tan x dx = sec x + C
∫ csc2 x dx = −cot x + C
∫ csc x cot x dx = −csc x + C
Ex. ∫ (3 cos x − 2 csc x cot x )dx = 3∫ cos x dx − 2∫ csc x cot x dx
= 3 sin x − 2(− csc x ) + C = 3 sin x + 2 csc x + C
Initial value problems
If given some extra condition, a particular antiderivative of a given function
can then be found. That is, we can find the value of the constant of
integration that satisfies the extra condition and thus uniquely determine an
antiderivative of the function. A common type of such additional condition
is called the initial condition or initial value.
Ex. Find the antiderivative F(x) of f (x) = 6x2 + 5, such that F(1) = 2.
1 
F ( x) = ∫ (6 x 2 + 5) dx = 6 x 3  + 5 x + C = 2 x 3 + 5 x + C
3 
F(1) = 2 = 2(13) + 5(1) + C
2 = 7 + C → C = −5
Therefore, F(x) = 2x3 + 5x − 5.
Ex. Find y if y″ = 1 − sin x, y(0) = 1 and y′(0) = −3.
y′ = ∫ y″ dx = ∫ (1 − sin x) dx = x + cos x + C
y′(0) = −3 = 0 + cos(0) + C = 1 + C
−3 = 1 + C → C = −4
→ y′ = x + cos x − 4
x2
y = ∫ y ′ dx = ∫ ( x + cos x − 4) dx =
+ sin x − 4 x + C1
2
y(0) = 1 = 0 + sin(0) − 0 + C1 = C1 → C1 = 1
x2
+ sin x − 4 x + 1
→ y=
2
Ex. A ball is tossed straight up from the top of a 30-meter building with an
initial velocity of 20 m/sec. Find the position function describing its height.
Known: a(t) = −9.8 (m/sec2), v(0) = 20 (m/sec), s(0) = 30 (m).
v(t) = ∫ a(t) dt = ∫ −9.8 dt = −9.8t + C
v(0) = 20 = −9.8(0) + C = C → C = 20
v(t) = −9.8t + 20
s(t) = ∫ v(t) dt = ∫ (−9.8t + 20) dt = −4.9t2 + 20t + C1
s(0) = 30 = −4.9(0) + 20(0) + C1 = C1
C1 = 30
Therefore, s(t) = −4.9t2 + 20t + 30 (meters)
The above example can be easily generalized to obtain the formula from the
physics class describing the height of a free-falling object that was tossed
straight up in the air with an initial velocity of v0 and an initial height of s0:
In the metric system:
s(t) = −4.9t2 + v0 t + s0 (meters)
In the English system:
s(t) = −16t2 + v0 t + s0 (feet)
```
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