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Answers for “如何简化 $3^{3/2}$?”

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Meet the concept

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The antiderivative of u\sqrt{u} is found by rewriting the square root as a fractional power, u1/2u^{1/2}, and then applying the power rule for integration. The power rule states that ∫undu=un+1n+1\int u^n du = \frac{u^{n+1}}{n+1}.

Conditions: The integrand is u\sqrt{u}, which is equivalent to u1/2u^{1/2}.; The power rule for integration is applicable.; u≥0u \ge 0 for the real-valued square root form.

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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Factoring and cancelling resolves the limit by simplifying the expression to a form that is defined near the target point. For x≠2x \ne 2 near 2, the expression x2+3x−10x2−2x\frac{x^2+3x-10}{x^2-2x} factors into (x−2)(x+5)x(x−2)\frac{(x-2)(x+5)}{x(x-2)}.

Conditions: The limit is as x→2x \to 2.; The original expression is undefined at x=2x=2 (indeterminate form 0/00/0).; Cancellation is valid only for x≠2x \ne 2.

Understand why

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The presenter factors out the common coefficient 23\frac{2}{3} to make the subsequent arithmetic simplification easier. After evaluating the antiderivative at the upper and lower bounds, both terms inside the brackets contain the factor 23\frac{2}{3}.

Conditions: The expression to evaluate is 49[23⋅93/2−23⋅13/2]\frac{4}{9}\left[\frac{2}{3}\cdot 9^{3/2}-\frac{2}{3}\cdot 1^{3/2}\right].; Both bracketed terms contain the same factor 23\frac{2}{3}.

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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Clearing the inner fraction resolves the limit by simplifying the complex rational expression into a polynomial form. Near x=−1x=-1, with x≠−1x\ne-1, multiplying the numerator and denominator of 3+3x1/x+1\frac{3+3x}{1/x+1} by xx gives 3x+3x21+x\frac{3x+3x^2}{1+x}.

Conditions: The limit is as x→−1x \to -1.; The expression contains an inner fraction 1/x1/x in the denominator.; Multiplication by xx is valid for x≠0x \ne 0, and specifically near x=−1x=-1 where x≠0x \ne 0.; The original expression is undefined at x=−1x=-1 (indeterminate form 0/00/0).

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.