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The line \(y = x - 1\) can be expressed in polar coordinates \((r, \theta)\) as

  • A.\(r = \cos\theta\)
  • B.\(r = \sin\theta\)
  • C.\(r(\cos\theta + \sin\theta) = 1\)
  • D.\(r(\cos\theta - \sin\theta) = 1\)

Answer: D

The line is \(r(\cos\theta - \sin\theta) = 1\).

Substitute \(x = r\cos\theta\) and \(y = r\sin\theta\) into \(y = x - 1\):

\[ \begin{aligned} r\sin\theta &= r\cos\theta - 1 \\ r\cos\theta - r\sin\theta &= 1 \\ r(\cos\theta - \sin\theta) &= 1 \end{aligned} \]

The option with a plus sign is the line \(x + y = 1\), which has slope \(-1\), not \(+1\). The options \(r = \cos\theta\) and \(r = \sin\theta\) are circles through the origin (\(x^{2} + y^{2} = x\) and \(x^{2} + y^{2} = y\)), not straight lines.

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