# CHAPTER 4 SECTION 4.2 AREA.

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CHAPTER 4 SECTION 4.2 AREA

Sigma (summation) notation REVIEW
In this case k is the index of summation The lower and upper bounds of summation are 1 and 5 In this case i is the index of summation The lower and upper bounds of summation are 1 and 6

Sigma notation

Sigma Summation Notation

Practice with Summation Notation
= 3080

Practice with Summation Notation
Numerical Problems can be done with the TI83+/84 as was done in PreCalc Algebra Sum is in LIST, MATH Seq is on LIST, OPS

Area Under a Curve by Limit Definition
The area under a curve can be approximated by the sum of rectangles. The figure on the left shows inscribed rectangles while the figure on the right shows circumscribed rectangles This gives the lower sum. This gives the upper sum.

Left endpoint approximation:
Approximate area: (too low)

Right endpoint approximation:
Approximate area: (too high) Averaging the right and left endpoint approximations: (closer to the actual value)

Approximating definite integrals: different choices for the sample points
If xi* is chosen to be the left endpoint of the interval, then xi* = xi-1 and we have If xi* is chosen to be the right endpoint of the interval, then xi* = xi and we have Ln and Rn are called the left endpoint approximation and right endpoint approximation , respectively.

Can also apply midpoint approximation: choose the midpoint of the subinterval as the sample point. Approximate area: The midpoint rule gives a closer approximation than the trapezoidal rule, but in the opposite direction.

Midpoint rule

Approximating the Area of a Plane Region
b. y y f(x) = -x2 + 5 f(x) = -x2 + 5 5 5 4 4 3 3 2 2 1 1 x x 2/ / / / 2/5 4/5 6/ / To approximate the area under each curve, you must sum the area of each rectangle. See next slide

The right endpoints, Mi, of the intervals are 2i/5, where i = 1, 2, 3, 4, 5. The width of each rectangle is 2/5 and the height of each rectangle can be obtained by evaluating f at the right endpoint of each interval. [0, 2/5], [2/5, 4/5], [4/5, 6/5], [6/5, 8/5], [8/5, 10/5] Evaluate f(x) at the right endpoints of each of these intervals. The sum of the area of the five rectangles is Height Width Because each of the five rectangles lies inside the parabolic region, you can conclude that the area of the parabolic region is greater than 6.48.

Approximating the Area of a Plane Region for b (con’t)
b. The left endpoints of the five intervals are 2/5(i _ 1), where i = 1, 2, 3, 4, 5. The width of each rectangle is 2/5, and the height of each rectangle can be found by evaluating f at the left endpoint of each interval. Height Width Because the parabolic region lies within the union of the five rectangular region, that the area of the parabolic region is less than 8.08. 6.48 < Area of region < 8.08

ON CALCULATOR

In general for the upper sum S(n) and Lower sum s(n), you use the following for curves f(x) bound between x=a and x=b.

Finding Upper and Lower Sums for a Region
Find the upper and lower sums for the region bounded by the graph of f(x) = x2 and the x-axis between x = 0 and x = 2 Solution Begin by partitioning the interval [0, 2] into n subintervals, each of length A. B. f(x) = x2 f(x) = x2 4 4 3 3 2 2 1 1

Left endpoints Right endpoints

Limit of the Lower and Upper Sums

Definition of the Area of a Region in the Plane

Area Under a Curve by Limit Definition
If the width of each of n rectangles is x, and the height is the minimum value of f in the rectangle, f(Mi), then the area is the limit of the area of the rectangles as n  This gives the lower sum.

Area under a curve by limit definition
If the width of each of n rectangles is x, and the height is the maximum value of f in the rectangle, f(mi), then the area is the limit of the area of the rectangles as n  This gives the upper sum.

Area under a curve by limit definition
The limit as n  of the Upper Sum = The limit as n  of the Lower Sum = The area under the curve between x = a and x = b.

Theorem 4.3 Limits of the Lower and Upper Sums

Definition of the Area of a Region in the Plane

Visualization f(ci) ci Width = Δx ith interval

Example: Area under a curve by limit definition
Find the area of the region bounded by the graph f(x) = 2x – x3 , the x-axis, and the vertical lines x = 0 and x = 1, as shown in the figure.

Area under a curve by limit definition
Why is right, endpoint i/n? Suppose the interval from 0 to 1 is divided into 10 subintervals, the endpoint of the first one is 1/10, endpoint of the second one is 2/10 … so the right endpoint of the ith is i/10.

Visualization again f(ci) ci = i/n Width = Δx= ith interval

Sum of all the rectangles
Find the area of the region bounded by the graph f(x) = 2x – x3 on [0, 1] Sum of all the rectangles Right endpoint Sub for x in f(x) Use rules of summation

…continued Foil & Simplify

The area of the region bounded by the graph f(x) = 2x – x3 , the x-axis, and the vertical lines x = 0 and x = 1, as shown in the figure = .75 0.75

Practice with Limits Multiply out Separate