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Answers for “如何积分 $\sqrt{x}$?”

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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 ∫−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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To evaluate ∫23xdx\int_2^3 \sqrt{x} dx, first find the antiderivative 23x3/2\frac{2}{3}x^{3/2}. Then, apply the Fundamental Theorem of Calculus by subtracting the value of the antiderivative at the lower bound from its value at the upper bound: [23x3/2]23=23(3)3/2−23(2)3/2\left[\frac{2}{3}x^{3/2}\right]_2^3 = \frac{2}{3}(3)^{3/2} - \frac{2}{3}(2)^{3/2}.

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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To use the curve y2=2xy^2 = 2x with vertical strips, solve for yy by taking the square root, yielding y=±2xy = \pm\sqrt{2x}. Since the region lies in the first quadrant where y≥0y \ge 0, select the positive root y=2x1/2y = \sqrt{2} x^{1/2} to define the strip height as a function of xx.

Conditions: The boundary curve is given as y2=2xy^2 = 2x.; The region is in the first quadrant (x≥0,y≥0x \ge 0, y \ge 0).; Vertical strips are being used, requiring height as a function of xx.

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To evaluate the definite integral, substitute x=1+eux = 1 + e^u, which gives dx=eududx = e^u du. Convert the bounds from u=0u=0 and u=ln⁡2u=\ln 2 to x=2x=2 and x=3x=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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The substitution x=1+eux = 1 + e^u simplifies the integrand because its derivative dx=eududx = e^u du exactly matches the eudue^u du term in the original integral. This allows the complex expression eu1+eudue^u \sqrt{1+e^u} du to be rewritten as the simpler radical xdx\sqrt{x} dx.

Conditions: The integrand contains eu1+eue^u \sqrt{1+e^u}.; The substitution is x=1+eux = 1 + e^u.; The differential is dx=eududx = e^u du.

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The substitution works because the derivative of 1+x41+x^4 is 4x34x^3, which matches the x3x^3 term present in the integrand x31+x4x^3\sqrt{1+x^4}. This allows the integral to be transformed into a simpler power function form, enabling the calculation of the antiderivative required for the integration by parts step.

Conditions: The term to be integrated is x31+x4x^3\sqrt{1+x^4}.; The substitution variable is w=1+x4w = 1+x^4.; The differential dwdw is proportional to x3dxx^3 dx.

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To set up the integral for the area moment of inertia about the y-axis (IyI_y) using a vertical strip, express the differential area dAdA as the product of the strip's height (upper boundary minus lower boundary) and its width dxdx. Substitute this into the definition Iy=∫x2dAI_y = \int x^2 dA and determine the integration limits based on the horizontal extent of the region bounded by functions of xx.

Conditions: The region is bounded by curves that can be expressed as functions of xx.; A vertical differential strip is chosen, meaning its width is dxdx.; The integration variable is xx.

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To set up the definite integral for the arc length of the curve y=x3/2y = x^{3/2} over the interval [0,32/9][0, 32/9], you first apply the power rule to find the derivative of the function, which is f′(x)=32x1/2f'(x) = \frac{3}{2}x^{1/2}. Next, you square this derivative to get (f′(x))2=94x(f'(x))^2 = \frac{9}{4}x.

Conditions: The curve is defined by the function f(x)=x3/2f(x) = x^{3/2}.; The interval of integration is [0,32/9][0, 32/9].; The arc length formula ∫ab1+(f′(x))2dx\int_{a}^{b} \sqrt{1 + (f'(x))^2} dx is applicable.

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To differentiate an integral with a square-root upper limit, you evaluate the integrand at the upper limit and multiply by the derivative of that limit. For an upper limit u=tu=\sqrt{t}, the derivative is 1/(2t)1/(2\sqrt{t}).

Conditions: The integrand is continuous on the relevant interval.; The upper limit is a differentiable function of tt, specifically u=tu=\sqrt{t}.; t>0t > 0.

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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.