# 5.2 Definite Integrals In this section we move beyond finite sums to see what happens in the limit, as the terms become infinitely small and their number.

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5.2 Definite Integrals In this section we move beyond finite sums to see what happens in the limit, as the terms become infinitely small and their number infinitely large. Sigma notation enables us to express a large sum in compact form:

Definite Integrals The Greek capital letter, sigma, stands for “sum”.
The index k tells us where to begin the sum (at the number below the sigma) and where to end (at the number above the sigma). If the symbol, infinity, appears above the sigma, it indicates that the terms go on indefinitely. These sums are called Riemann sums. LRAM, MRAM, and RRAM are examples of Riemann sums – not because they estimated area, but because they were constructed in a particular way.

Definite Integrals Figure 5.12 is a continuous function f(x) defined on a closed integral [a , b]. It may have negative values as well as positive values.

Definite Integrals To make the notation consistent, we denote a by x0 and b by xn. The set P = {x0, x1, x2, …, xn} is called a partition of [a , b]. The partition P determines n closed subintervals. The kth subinterval is [xk – 1 , xk], which has length xk = xk – xk – 1.

Definite Integrals The value of the definite integral of a function over any particular interval depends on the function and not on the letter we choose to represent its independent variable. If we decide to use t or u instead of x, we simply write the integral as: No matter how we represent the integral, it is the same number, defined as a limit of Riemann sums. Since it does not matter what letter we use to run from a to b, the variable of integration is called a dummy variable.

Using the Notation The interval [-1 , 3] is partitioned into n subintervals of equal length Let mk denote the midpoint of the kth subinterval. Express the limit as an integral.

Revisiting Area Under a Curve
Evaluate the integral

If an integrable function y = f(x) is nonpositive, the nonzero terms in the Riemann sums for f over an interval [a , b] are negatives of rectangle areas. The limit of the sums, the integral of f from a to b, is therefore the negative of the area of the region between the graph of f and the x-axis.

If an integrable function y = f(x) has both positive and negative values on an interval [a , b], then the Riemann sums for f on [a , b] add areas of rectangles that lie above the x-axis to the negatives of areas of rectangles that lie below the x-axis. The resulting cancellations mean that the limiting value is a number whose magnitude is less than the total area between the curve and the x-axis. The value of the integral is the area above the x-axis minus the area below.

Constant Functions Integrals of constant functions are easy to evaluate. Over a closed interval, they are simply the constant times the length of the interval (Figure 5.21).

Revisiting the Train Problem
A train moves along a track at a steady 75 miles per hour from 7:00 A.M. to 9:00 A.M. Express its total distance traveled as an integral. Evaluate the integral using Theorem 2.

Using NINT Evaluate the following integrals numerically.

More Practice!!!!! Homework – Textbook p. 282 – 283 #1 – 22 ALL.

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