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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.

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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.

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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.

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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.

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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.