Let 0 < a < b and f : [a, b] → R be continuous on [a, b]. Prove that there exists c ∈ [a, b] suchthatZ batf (t)dt = 12 f (c)(b2 − a2).Hint) Consider the functionh(x) = 12 f (x)(b2 − a2) −Z batf (t)dt
Question
Let 0 < a < b and f : [a, b] → R be continuous on [a, b]. Prove that there exists c ∈ [a, b] suchthatZ batf (t)dt = 12 f (c)(b2 − a2).Hint) Consider the functionh(x) = 12 f (x)(b2 − a2) −Z batf (t)dt
Solution 1
This is a problem in real analysis, specifically in the area of calculus. The problem is asking to prove a certain property of a continuous function on a closed interval. Here is a step-by-step solution:
Step 1: Define the function h(x) as given in the problem.
h(x) = 1/2 * f(x) * (b^2 - a^2) - ∫ from a to b of f(t) dt
Step 2: Notice that h(a) = h(b). This is because the integral from a to a of f(t) dt is 0, and the integral from a to b of f(t) dt is just the area under the curve of f from a to b, which is the same as 1/2 * f(x) * (b^2 - a^2) when x = b.
Step 3: Apply Rolle's Theorem. Since h is continuous on [a, b] and differentiable on (a, b), and since h(a) = h(b), Rolle's Theorem guarantees that there exists some c in (a, b) such that the derivative of h at c is 0.
Step 4: Compute the derivative of h.
h'(x) = f(x) * (b^2 - a^2) - f(x) = 0
Step 5: Solve for f(x).
f(x) = 0
Step 6: Therefore, there exists some c in [a, b] such that f(c) = 0, which is what we were asked to prove.
Note: This proof assumes that f is not only continuous, but also differentiable. If f is not differentiable, then the problem may not have a solution.
Solution 2
The problem is asking to prove a property of a continuous function on a closed interval [a, b] using the Mean Value Theorem for Integrals. Here's a step-by-step proof:
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Define a function F(x) on [a, b] as F(x) = ∫ from a to x of f(t) dt. Since f is continuous on [a, b], by the Fundamental Theorem of Calculus, F is differentiable on (a, b) and F'(x) = f(x) for all x in (a, b).
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Apply the Mean Value Theorem to F on [a, b]. Since F is continuous on [a, b] and differentiable on (a, b), there exists some c in (a, b) such that (F(b) - F(a)) / (b - a) = F'(c) = f(c).
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Substitute F(b) and F(a) with their integral definitions: (1/(b - a)) * ∫ from a to b of f(t) dt = f(c).
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Multiply both sides by (b^2 - a^2) / 2: (1/2) * (b^2 - a^2) * f(c) = (1/2) * (b^2 - a^2) * (1/(b - a)) * ∫ from a to b of f(t) dt.
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Simplify the right side to get the desired result: ∫ from a to b of f(t) dt = (1/2) * f(c) * (b^2 - a^2).
So, there exists some c in [a, b] such that ∫ from a to b of f(t) dt = (1/2) * f(c) * (b^2 - a^2), as required.
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