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Approximating Area under a curve with rectangles To find the area under a curve we approximate the area using rectangles and then use limits to find the area.
Example 1 Suppose we want to estimate A = the area under the curve y = 1 − x^2 , 0 ≤ x ≤ 1.
2
1
f ( x ) = 1 – x^2
Left endpoint approximation To approximate the area under the curve, we can circumscribe the curve using rectangles as follows:
- We divide the interval [0, 1] into 4 subintervals of equal length, ∆x = 1 − 4 0 = 1/4. This divides the interval [0, 1] into 4 subintervals [0, 1 /4], [1/ 4 , 1 /2], [1/ 2 , 3 /4], [3/ 4 , 1] each with length ∆x = 1/4. We label the endpoints of these subintervals as x 0 = 0, x 1 = 1/ 4 , x 2 = 2/ 4 , x 3 = 3/ 4 , x 4 = 1.
- Above each subinterval draw a rectangle with height equal to the height of the function at the left end point of the subinterval. The values of the function at the endpoints of the subintervals are xi x 0 = 0 x 1 = 1/ 4 x 2 = 1/ 2 x 3 = 3/ 4 x 4 = 1 f (xi) = 1 − x^2 i 1 15 / 16 3 / 4 7 / 16 0
- We use the sum of the areas of the approximating rectangles to approximate the area under the curve. We get
A ≈ L 4 =
∑^4
i=
f (xi− 1 )∆x = f (x 0 ) · ∆x + f (x 1 ) · ∆x + f (x 2 ) · ∆x + f (x 3 ) · ∆x =
L 4 is called the left endpoint approximation or the approximation using left endpoints (of the subin- tervals) and 4 approximating rectangles. We see in this case that L 4 = 0. 78125 > A (because the function is decreasing on the interval).
There is no reason why we should use the left end points of the subintervals to define the heights of the approximating rectangles, it is equally reasonable to use the right end points of the subintervals, or the midpoints or in fact a random point in each subinterval.
Right endpoint approximation In the picture on the left above, we use the right end point to define the height of the approximating rectangle above each subinterval, giving the height of the rectangle above [xi− 1 , xi] as f (xi). This gives us inscribed rectangles. The sum of their areas gives us The right endpoint approximation, R 4 or the approximation using 4 approximating rectangles and right endpoints. Use the table above to complete the calculation:
A ≈ R 4 =
∑^4
i=
f (xi)∆x = f (x 1 ) · ∆x + f (x 2 ) · ∆x + f (x 3 ) · ∆x + f (x 4 ) · ∆x =
Is R 4 less than A or greater than A.
Midpoint Approximation In the picture in the center above, we use the midpoint of the intervals to define the height of the approximating rectangle. This gives us The Midpoint Approximation or The Midpoint Rule:
xmi = midpoint xm 1 = 0.125 = 1/ 8 xm 2 = 0.375 = 3/ 8 xm 3 = 0.625 = 5/ 8 xm 4 = 0.625 = 7/ 8 f (xmi ) = 1 − (xmi )^2 63 / 64 55 / 64 39 / 64 15 / 64
A ≈ M 4 =
∑^4
i=
f (xmi )∆x = f (xm 1 )∆x + f (xm 2 )∆x + f (xm 3 )∆x + f (xm 4 )∆x =
General Riemann Sum We can use any point in the interval x∗ i ∈ [xi− 1 , xi] to define the height of the corresponding approximating rectangle as f (x∗ i ). In the picture on the right we use random points
Calculating Limits of Riemann sums The following formulas are sometimes useful in calculating Riemann sums. I have attached some visual proofs at the end of the lecture.
∑^ n
i=
i =
n(n + 1) 2
∑^ n
i=
i^2 =
(2n + 1)n(n + 1) 6
∑^ n
i=
i^3 =
[
n(n + 1) 2
] 2
Let us now consider Example 1. We want to find A = the area under the curve y = 1 − x^2 on the interval [a, b] = [0, 1].
2
1
f ( x ) = 1 – x^2
We know that A = limn→∞ Rn, where Rn is the right endpoint approximation using n approximating rectangles. We must calculate Rn and than find limn→∞ Rn.
- We divide the interval [0, 1] into n strips of equal length ∆x = 1 − n^0 = 1/n. This gives us a partition of the interval [0, 1],
x 0 = 0, x 1 = 0 + ∆x = 1/n, x 2 = 0 + 2∆x = 2/n,... , xn− 1 = (n − 1)/n, xn = 1.
- We will use the right endpoint approximation Rn.
- The heights of the rectangles can be found from the table below:
xi x 0 = 0 x 1 = 1/n x 2 = 2/n x 3 = 3/n... xn = n/n
f (xi) = 1 − (xi)^2 1 1 − 1 /n^2 1 − 22 /n^2 1 − 32 /n^2... 1 − n^2 /n^2
Rn = f (x 1 )∆x + f (x 2 )∆x + f (x 3 )∆x + · · · + f (xn)∆x = ( 1 −
n^2
n
n^2
n
n^2
n
n^2 n^2
n
n
n^2
n
n
n^2
n
n
n^2
n
n
n^2 n^2
n
- Finish the calculation above and find A = limn→∞ Rn using the formula for the sum of squares and calculating the limit as if Rn were a rational function with variable n.
Also A = limn→∞ Ln
From Part 3, we have ∆x = 1/n and
Ln =
n
n^2
n
n^2
n
n^2
n
(n − 1)^2 n^2
n
1 n
n
n^2
n
n
n^2
n
n
n^2
n
n
(n − 1)^2 n^2
n
grouping the (^) n^1 ’s together, we get
n n
n
[ 12
n^2
n^2
n^2
(n − 1)^2 n^2
]
n^3
[
12 + 2^2 + 3^2 + · · · + (n − 1)^2
]
n^3
n∑− 1
i=
i^2 = 1 −
n^3
[(2(n − 1) + 1)(n − 1)((n − 1) + 1)
6
]
n^3
[(2n − 1)(n − 1)(n)
6
]
n 6 n^3
(2n − 1)(n − 1)
2 n + smaller powers of n 6 n^2 So lim n→∞ Ln = lim n→∞
[
2 n + smaller powers of n 6 n^2
]
Appendix Area under a curve, Summary of method using Riemann sums. To find the area under the curve y = f (x) on the interval [a, b], where f (x) ≥ 0 for all x in [a, b] and continuous on the interval:
- Divide the interval into n strips of equal width ∆x = b−n a. This divides the interval [a, b] into n subintervals: [x 0 = a, x 1 ], [x 1 , x 2 ], [x 2 , x 3 ],... , [xn− 1 , xn = b]. (Note x 1 = a+∆x, x 2 = a+2∆x, x 3 = a+3∆x,... xn− 1 = a+(n−1)∆x, xn = a+n∆x = b.)
- For each interval, pick a sample point, x∗ i in the interval [xi− 1 , xi].
- Construct an approximating rectangle above the subinterval [xi− 1 , xi] with height f (x∗ i ). The area of this rectangle is f (x∗ i )∆x.
- The total area of the approximating rectangles is
f (x∗ 1 )∆x + f (x∗ 2 )∆x + · · · + f (x∗ n)∆x
(The sum is called a Riemann Sum.)
- We define the area under the curve y = f (x) on the interval [a, b] as
A = lim n→∞ [f (x∗ 1 )∆x + f (x∗ 2 )∆x + · · · + f (x∗ n)∆x]
= lim n→∞ Rn = lim n→∞ [f (x 1 )∆x + f (x 2 )∆x + · · · + f (xn)∆x]
= lim n→∞ Ln = lim n→∞ [f (x 0 )∆x + f (x 1 )∆x + · · · + f (xn− 1 )∆x].
(In a more advanced course, you would prove that all of these limits give the same number A which we use as a measure/definition of the area under the curve)
Extra Example Estimate the area under the graph of f (x) = 1/x from x = 1 to x = 4 using six approximating rectangles and
∆x = b−na = , where [a, b] = [1, 4] and n = 6. Mark the points x 0 , x 1 , x 2 ,... , x 6 which divide the interval [1, 4] into six subintervals of equal length on the following axis:
Fill in the following tables:
xi x 0 = x 1 = x 2 = x 3 = x 4 = x 5 = x 6 =
f (xi) = 1/xi
(a) Find the corresponding right endpoint approximation to the area under the curve y = 1/x on the interval [1, 4].
R 6 =
(b) Find the corresponding left endpoint approximation to the area under the curve y = 1/x on the interval [1, 4].
L 6 =
(c) Fill in the values of f (x) at the midpoints of the subintervals below:
midpoint = xmi xm 1 = xm 2 = xm 3 = xm 4 = xm 5 = xm 6 = f (xmi ) = 1/xmi
Find the corresponding midpoint approximation to the area under the curve y = 1/x on the interval [1, 4].
M 6 =
Extra Example Find the area under the curve y = x^3 on the interval [0, 1]. We know that A = limn→∞ Rn, where Rn is the right endpoint approximation using n approximating rectangles. We must calculate Rn and than find limn→∞ Rn.
- We divide the interval [0, 1] into n strips of equal length ∆x = 1 − n^0 = 1/n. This gives us a partition of the interval [0, 1],
x 0 = 0, x 1 = 0 + ∆x = 1/n, x 2 = 0 + 2∆x = 2/n,... , xn− 1 = (n − 1)/n, xn = 1.
- We will use the right endpoint approximation Rn.
- The heights of the rectangles can be found from the table below:
xi x 0 = 0 x 1 = 1/n x 2 = 2/n x 3 = 3/n... xn = n/n
f (xi) = (xi)^3 0 1 /n^3 23 /n^3 33 /n^3... n^3 /n^3
Rn = f (x 1 )∆x + f (x 2 )∆x + f (x 3 )∆x + · · · + f (xn)∆x = ( (^1)
n^3
n
n^3
n
n^3
n
(n 3
n^3
n
∑^ n
i=
i^3 n^4
n^4
∑^ n
i=
i^3 =
n^4
[
n(n + 1) 2
] 2
A = lim n→∞
n^4
[
n(n + 1) 2
] 2
= lim n→∞
n^2 (n + 1)^2 4 n^4
= lim n→∞
(n + 1)^2 4 n^2
= lim n→∞
(n + 1) n
(n + 1) n
Extra Example, estimates from data on rate of change The same principle applies to estimating Volume from discrete data on its rate of change: Oil is leaking from a tanker damaged at sea. The damage to the tanker is worsening as evidenced by the increased leakage each hour, recorded in the following table.
time in h 0 1 2 3 4 5 6 7 8
leakage in gal/h 50 70 97 136 190 265 369 516 720
The following gives the right endpoint estimate of the amount of oil that has escaped from the tanker after 8 hours:
R 8 = 70 · 1 + 97 · 1 + 136 · 1 + 190 · 1 + 265 · 1 + 369 · 1 + 516 · 1 + 720 · 1 = 2363 gallons.
The following gives the right endpoint estimate of the amount of oil that has escaped from the tanker after 8 hours:
L 8 = 50 · 1 + 70 · 1 + 97 · 1 + 136 · 1 + 190 · 1 + 265 · 1 + 369 · 1 + 516 · 1 = 1693 gallons.
Since the flow of oil seems to be increasing over time, we would expect that L 8 < true volume leaked < R 8 or the true volume leaked in the first 8 hours is somewhere between 1693 and 2363 gallons.
Visual proof of formula for the sum of integers:
