Calculus help: Evaluate the limit of Riemann sums for x^2-3x+4
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Calculus help: Evaluate the limit of Riemann sums for x^2-3x+4

[From: ] [author: ] [Date: 12-04-30] [Hit: ]
6/3+8/3= 14/3= 4.EDIT: Sorry, I had an arithmetic error! Its fixed.......
The problem is from the integral 0 to 2 (x^2-3x+4)dx as a limit of Riemann sums. Then evaluate the limit.
I tried working the problem out but I keep getting the wrong answer, which in turn gave me the wrong limit. Can someone please help me figure this out.

Any help is appreciated!!!!!

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Let's use right Riemann sums (we could use left Riemann, but I prefer right sums)

Interval: [0, 2]

Width of subintervals: (2−0)/n = 2/n
xi = 0 + 2/n * i = 2i/n , for i = 1 to n

Sn = ∑[i=1 to n] 2/n * f(2i/n)
Sn = 2/n ∑[i=1 to n] ((2i/n)² − 3(2i/n) + 4)
Sn = 2/n ∑[i=1 to n] (4i²/n² − 6i/n + 4)
Sn = 2/n { 4/n² ∑[i=1 to n] (i²) − 6/n ∑[i=1 to n] (i) + 4 ∑[i=1 to n] (1) }
Sn = 2/n { 4/n² * n(n+1)(2n+1)/6 − 6/n * n(n+1)/2 + 4 * n }
Sn = 4(n+1)(2n+1)/(3n²) − 6(n+1)/n + 8

Now we could use a common denominator and simplify, but this is likely to lead to calculation errors, and take up time. Instead we can just find limit as n approaches infinity now

lim[n→∞] {4(n+1)(2n+1)/(3n²) − 6(n+1)/n + 8}
= lim[n→∞] {4(n+1)(2n+1)/(3n²)} − lim[n→∞] {6(n+1)/(2n)} + lim[n→∞] {8}
= 8/3 − 6 + 8
= 14/3

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Integral (0 > 2)

(If you are not understanding how to integrate: Add one to the exponent of the x and then put the reciprocal in the front. For example, x^2. add one to the two > x^3. Then multiply it by its reciprocal which is 1/3. So it becomes 1/3x^3. If there is a number already in front, you can just combine it all together. But you ALWAYS put the reciprocal in the front.)

Integrated: 1/3x^3-3/2x^2+4x (indefinite)

Evaluated from 0 to 2: 1/3(2)^3-3/2(2)^2+4(2) - 1/3(0)^3-3/2(0)^2+4(0)

8/3-6+8-0

2+8/3

6/3+8/3= 14/3= 4.66

EDIT: Sorry, I had an arithmetic error! Its fixed.
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