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Answers for “如何计算从 2 到 3 的 sqrt(x) 定积分?”

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To evaluate ∫23xdx\int_2^3 \sqrt{x} dx, find the antiderivative 23x3/2\frac{2}{3}x^{3/2} and apply the Fundamental Theorem of Calculus. Subtract the value at the lower bound from the value at the upper bound to obtain the simplified result 23−4232\sqrt{3} - \frac{4\sqrt{2}}{3}.

Conditions: The integral is ∫23xdx\int_2^3 \sqrt{x} dx.; The antiderivative is 23x3/2\frac{2}{3}x^{3/2}.; The bounds are 2 and 3.

Find a method

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The definite integral can be evaluated by recognizing that the integrand y=9−x2y = \sqrt{9-x^2} graphs as an upper semicircle of radius 3. Because the function is continuous and nonnegative on [−3,3][-3, 3], the integral equals the ordinary geometric area of this region.

Conditions: The integrand is recognized as the upper semicircle of x2+y2=9x^2+y^2=9.; The function is continuous and nonnegative on the interval [−3,3][-3, 3].; Use the real geometric area formula for a circle.

Understand why

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The equation x2+y2=9x^2 + y^2 = 9 describes a full circle centered at the origin with radius 3, which includes both upper and lower branches. However, the original function is defined using the principal square root, y=9−x2y = \sqrt{9-x^2}.

Conditions: Working over the real numbers.; Using the principal (nonnegative) square root convention.; The underlying relation is the circle x2+y2=9x^2 + y^2 = 9.

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To evaluate the definite integral ∫0ln⁡2eu1+eu du\int_0^{\ln 2} e^u \sqrt{1+e^u} \, du, use the substitution x=1+eux = 1 + e^u, which yields dx=eu dudx = e^u \, du. Convert the limits of integration from u=0u=0 and u=ln⁡2u=\ln 2 to x=2x=2 and x=3x=3, transforming the integral into ∫23x dx\int_2^3 \sqrt{x} \, dx. Evaluating the antiderivative 23x3/2\frac{2}{3}x^{3/2} at these new bounds gives the result 23−4232\sqrt{3} - \frac{4\sqrt{2}}{3}.

Conditions: The integral is definite with bounds u=0u=0 to u=ln⁡2u=\ln 2.; The integrand is eu1+eue^u \sqrt{1+e^u}.; Substitution x=1+eux = 1 + e^u is used.

Meet the concept

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The antiderivative of x\sqrt{x} is 23x3/2\frac{2}{3}x^{3/2}, derived by rewriting the function as x1/2x^{1/2} and applying the power rule for integration. This result is specifically used to evaluate definite integrals after transforming the bounds, where the constant of integration is omitted.

Conditions: The function is x\sqrt{x}.; The power rule for integration is applied.; The exponent n=1/2n = 1/2.

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To evaluate the definite integral ∫−339−x2 dx\int_{-3}^{3} \sqrt{9-x^2}\,dx geometrically, recognize that the integrand y=9−x2y = \sqrt{9-x^2} represents the upper semicircle of a circle centered at the origin with radius 3. Because the function is nonnegative and continuous on the interval [−3,3][-3, 3], the definite integral equals the ordinary geometric area of this shaded region.

Conditions: The integrand is 9−x2\sqrt{9-x^2} and the limits of integration are -3 and 3.; The square root denotes the principal (nonnegative) root, restricting the graph to y≥0y \ge 0.; The function is continuous and nonnegative on the closed interval [−3,3][-3, 3], ensuring the definite integral equals the ordinary area under the curve.

Meet the concept

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The definite integral ∫0ln⁡2eu1+eudu\int_0^{\ln 2} e^u \sqrt{1+e^u} du evaluates to the exact value 23−4232\sqrt{3} - \frac{4\sqrt{2}}{3}. This result is obtained by applying the substitution x=1+eux = 1 + e^u, which transforms the bounds to 2 and 3, and then evaluating the antiderivative 23x3/2\frac{2}{3}x^{3/2} at these new limits.

Conditions: The integral is ∫0ln⁡2eu1+eudu\int_0^{\ln 2} e^u \sqrt{1+e^u} du.; The substitution x=1+eux = 1 + e^u is used.; The bounds are converted to 2 and 3.; The antiderivative is 23x3/2\frac{2}{3}x^{3/2}.

Meet the concept

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The real domain of the function y=9−x2y=\sqrt{9-x^2} is the closed interval [−3,3][-3, 3]. This restriction exists because, over the real numbers, the expression inside a square root (the radicand) must be nonnegative for the principal square root to be real-valued.

Conditions: Real-valued interpretation of the square root.; Using the principal square root convention.; The radicand is 9−x29 - x^2.

Meet the concept

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The final numerical value of the area moment of inertia IyI_y for the specific shaded region about the centroidal y-axis is approximately 0.762 m40.762 \text{ m}^4. This value is obtained by evaluating the definite integral ∫02x2(2−2x1/2)dx\int_0^2 x^2 (2 - \sqrt{2} x^{1/2}) dx, which simplifies to 243−2(27)23.5\frac{2^4}{3} - \sqrt{2} \left(\frac{2}{7}\right) 2^{3.5}.

Conditions: The region is bounded by y=2y = 2 (top) and y=2x1/2y = \sqrt{2} x^{1/2} (bottom) from x=0x=0 to x=2x=2.; The axis is the centroidal y-axis.; Units are in meters.

Understand why

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For a prime number pp, the only positive divisors are 1 and pp itself. Any integer aa in the range from 1 to pp that is not equal to pp will not share the factor pp.

Conditions: pp is a prime number.; φ(p)\varphi(p) is defined as the count of integers from 1 to pp that are coprime to pp.

Find a method

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When using u-substitution in a definite integral, the original limits of integration in terms of xx must be converted into new limits in terms of uu. This is done by substituting the original xx-bounds into the substitution equation u(x)u(x).

Conditions: The integral is a definite integral.; A substitution u=u(x)u = u(x) is being used.; The original limits of integration are given in terms of xx.

Understand why

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The substitution u=1+94xu = 1 + \frac{9}{4}x is chosen because it is exactly the expression under the radical in the arc-length integral ∫032/91+94x dx\int_0^{32/9}\sqrt{1+\frac{9}{4}x}\,dx. By setting uu to this inner expression, the complicated integrand simplifies to u\sqrt{u}, which is much easier to integrate using the power rule.

Conditions: The integral to evaluate is ∫032/91+94x dx\int_0^{32/9}\sqrt{1+\frac{9}{4}x}\,dx.; The integrand contains a composite expression under a square root.