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Answers for “如何积分 $\sqrt{x}$?”

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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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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 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.

Know when to use it

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These techniques are combined when the integrand can be split into a product where one factor simplifies upon differentiation (for integration by parts) and the other factor is integrable only via substitution. This often occurs with polynomials multiplied by composite functions like axn+b\sqrt{ax^n+b}.

Conditions: The integrand is a product of two distinct types of functions.; One part reduces nicely when differentiated.; The other part requires a u-substitution to find its antiderivative.

Find a method

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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.

Find a method

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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.

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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For the arc length formula ∫ab1+(f′(x))2dx\int_{a}^{b} \sqrt{1 + (f'(x))^2} dx to be valid, the function f(x)f(x) must be continuous on the closed interval [a,b][a, b], and its derivative f′(x)f'(x) must also be continuous on [a,b][a, b]. These conditions ensure that the curve is smooth enough for the integral to accurately represent its length.

Conditions: The curve is defined by a function y=f(x)y = f(x).; The interval of integration is [a,b][a, b].; f(x)f(x) is continuous on [a,b][a, b].; f′(x)f'(x) is continuous on [a,b][a, b].

Find a method

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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.

Meet the concept

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The curly d symbol (∂\partial) represents a partial derivative. It means taking the derivative of a multivariable function with respect to one specific variable at a time, while treating all other variables as constants.

Conditions: The function has more than one independent variable.; All variables except the differentiation variable are treated as constants during the partial derivative.

Understand why

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The factor is essential because the Fundamental Theorem of Calculus combined with the Chain Rule requires multiplying the integrand evaluated at the upper limit by the derivative of that limit. Without this factor, the rate of change of the accumulated area with respect to the outer variable tt would be incorrect.

Conditions: The upper limit is a function of tt, not just tt itself.; The integrand is continuous.

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.