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Answers for “如何使用 u-代换计算定积分 $\int_{0}^{32/9} \sqrt{1 + \frac{9}{4}x} dx$?”

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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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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].

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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.