Section 4.3 – Riemann Sums and Definite Integrals
Riemann Sums The rectangles need not have equal width, and the height may be any value of f(x) within the subinterval. 1. Partition (divide) [a,b] into N subintervals. 2. Find the length of each interval: 3. Find any point ci in the interval [xi,xi-1]. c1 c2 c3 ci cN =x0 a x1 x2 x3 xi =xN b 4. Construct every rectangle of height f(ci) and base Δxi. 4. Find the sum of the areas.
Riemann Sums The norm of P, denoted ││P││, is the maximum of the lengths Δxi. a b As ││P││ gets closer to 0, the sum of the areas of the rectangles is closer to the actual area under the curve
Riemann Sums The norm of P, denoted ││P││, is the maximum of the lengths Δxi. a b As ││P││ gets closer to 0, the sum of the areas of the rectangles is closer to the actual area under the curve
Upper limit of integration Lower limit of integration Definite Integral The definite integral of f(x) over [a,b], denoted by the integral sign, is the limit of Riemann sums: Where the limit exists, we say that f(x) is integrable over [a,b]. Upper limit of integration Lower limit of integration
Notation Examples The definite integral that represents the area is… EX1: f(x) S a b Ex2: The area under the parabola y=x2 from 0 to 1
Theorem: The Existence of Definite Integrals If f(x) is continuous on [a,b], or if f(x) is continuous with at most finitely many jump discontinuities (one sided limits are finite but not equal), then f(x) is integrable over [a,b]. a b
Negative Area or “Signed” Area If a function is less than zero for an interval, the region between the graph and the x-axis represents negative area. Positive Area Negative Area
Definite Integral: Area Under a Curve If y=f(x) is integrable over a closed interval [a,b], then the area under the curve y=f(x) from a to b is the integral of f from a to b. Upper limit of integration Lower limit of integration
Example 1 Calculate .
Example 2 Calculate .
Rules for Definite Integrals Let f and g be functions and x a variable; a, b, c, and k be constant. Constant Constant Multiple Sum Rule Reversing the Limits Additivity
Constant Multiple Rule Given and Constant Rule Example 1 If , calculate . Sum Rule Constant Multiple Rule Given and Constant Rule
Example 2 If , calculate . Additivity Rule Given