5 Steps to a 5 AP Calculus AB 2019 - William Ma

MA 3972-MA-Book April 11, 2018 15:57

Def inite Integrals 249

Example

Evaluate

∑n

i= 1

i(i+1)

n

.

Rewrite:

∑n

i= 1

i(i+1)

n

as

1

n

∑n

i= 1

(i^2 +i)=

1

n

( n

∑

i= 1

i^2 +

∑n

i= 1

i

)

=

1

n

(

n(n+1)(2n+1)

6

+

n(n+1)

2

)

=

1

n

[

n(n+1)(2n+1)+ 3 n(n+1)

6

]

=

(n+1)(2n+1)+3(n+1)

6

=

(n+1)[(2n+1)+ 3 ]

6

=

(n+1)(2n+4)

6

=

(n+1)(n+2)

3

.

(Note: This question has not appeared in an AP Calculus AB exam in recent years.)

TIP • Remember: In exponential growth/decay problems, the formulas are dy
dx
=kyand
y=y 0 ekt.

Definition of a Riemann Sum

Letfbe defined on [a,b] andxibe points on [a,b] such thatx 0 =a,xn=b, anda<x 1 <

x 2 <x 3 ···<xn− 1 <b. The pointsa, x 1 , x 2 , x 3 ,...xn− 1 , andbform a partition of f

denoted asΔon [a,b]. LetΔxibe the length of theith interval [xi− 1 , xi] andcibe any

point in theith interval. Then the Riemann sum off for the partition is

∑n

i= 1

f(ci)Δxi.

Example 1

Letf be a continuous function defined on [0, 12] as shown below.

x 0 2 4 6 8 10 12

f(x) 3 7 19 39 67 103 147

Find the Riemann sum for f(x) over [0, 12] with 3 subdivisions of equal length and the

midpoints of the intervals asci.

Length of an intervalΔxi=

12 − 0

3

=4. (See Figure 12.1-1.)