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For the circle \( x^{2} + y^{2} = 25 \), the slope \( dy/dx \) at the point \( (3, 4) \) is

  • A.\( 4/3 \)
  • B.\( -3/4 \)
  • C.\( 3/4 \)
  • D.\( -4/3 \)

Answer: B

Differentiate implicitly and solve for the slope:

\[ 2x + 2y\,\frac{dy}{dx} \;=\; 0 \quad \Longrightarrow \quad \frac{dy}{dx} \;=\; -\frac{x}{y}. \]

At \( (3,4) \) this is \( -3/4 \). The tangent is perpendicular to the radius, whose slope is \( 4/3 \), and the product of the two slopes is \( -1 \) as it must be

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