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Answers for “为什么主平方根 $\sqrt{9-x^2}$ 只给出圆 $x^2+y^2=9$ 的上半部分而不是整个圆?”

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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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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 ∫−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 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.

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.