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Answers for “如何积分 $\sqrt{x}$?”

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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.

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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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.

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.

Find a method

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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.

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.

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.

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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.

Understand why

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Rewriting the integrand separates it into a factor suitable for differentiation and a factor suitable for integration. Choosing x4x^4 for differentiation reduces its power, while the remaining factor x31+x4x^3\sqrt{1+x^4} becomes integrable via substitution because its derivative contains the necessary x3x^3 term.

Conditions: The integrand is x71+x4x^7\sqrt{1+x^4}.; Integration by parts is the chosen method.; A substitution is available for the second factor.

Understand why

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The derivative of 1+eu1 + e^u is eue^u, which appears as a factor in the integrand eu1+eue^u \sqrt{1+e^u}. This allows the entire term eudue^u du to be replaced by dxdx, simplifying the integral to ∫xdx\int \sqrt{x} dx.

Conditions: The integrand is eu1+eue^u \sqrt{1+e^u}.; The substitution is x=1+eux = 1 + e^u.; The derivative dx/du=eudx/du = e^u is present in the integrand.