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Answers for “$\sqrt{x}$ 的原函数是什么?”

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Meet the concept

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The antiderivative of x\sqrt{x} is 23x3/2\frac{2}{3}x^{3/2}. This is found by rewriting x\sqrt{x} as x1/2x^{1/2} and applying the power rule for integration, which states that ∫xndx=xn+1n+1\int x^n dx = \frac{x^{n+1}}{n+1} for n≠−1n \neq -1.

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

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.

Find a method

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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.

Understand why

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The factor x3x^3 is grouped with 1+x4\sqrt{1+x^4} because the derivative of the inner function 1+x41+x^4 is 4x34x^3. This presence of x3x^3 makes the expression x31+x4x^3\sqrt{1+x^4} directly integrable using substitution, which is a prerequisite for the integration by parts method.

Conditions: The integrand is x71+x4x^7\sqrt{1+x^4}.; The method involves integration by parts.; The term dvdv must be integrable.

Find a method

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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 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.

Meet the concept

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The final value is 23−4232\sqrt{3} - \frac{4\sqrt{2}}{3}. This is obtained by evaluating the antiderivative 23x3/2\frac{2}{3}x^{3/2} at the converted bounds x=3x=3 and x=2x=2 and subtracting the results.

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}.

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.

Find a method

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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.

Find a method

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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.

Meet the concept

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The exact value of the definite integral ∫−339−x2 dx\int_{-3}^{3} \sqrt{9-x^2}\,dx is 9π2\frac{9\pi}{2}. This is derived by interpreting the integral as the geometric area of the upper semicircle of a circle with radius 3.

Conditions: Interpret the integral as area under the graph on [−3,3][-3,3].; Recognize the graph as the upper semicircle of radius 3.; Exact equality for the displayed definite integral.