A piecewise linear function f(x) is plotted using thick solid lines in the…

2003

A piecewise linear function f(x) is plotted using thick solid lines in the figure below (the plot is drawn to scale).

cs-2003_clip_image032

If we use the Newton-Raphson method to find the roots of f(x) = 0 using x0, x1 and x2 respectively as initial guesses, the roots obtained would be

  1. A.

    1.3, 0.6, and 0.6 respectively

  2. B.

    0.6, 0.6, and 1.3 respectively

  3. C.

    1.3, 1.3, and 0.6 respectively

  4. D.

    1.3, 0.6, and 1.3 respectively

Show answer & explanation

Correct answer: D

For a piecewise linear graph, the Newton-Raphson tangent at a point on a linear segment is that segment itself. Thus the next iterate is the x-intercept of the line segment containing the current point. From x1, the point lies on the decreasing middle segment, whose x-intercept is the left root, approximately 0.6. From x2, the point lies on the increasing right segment, whose x-intercept is the right root, approximately 1.3. From x0, the first tangent is along the left segment and sends the iteration toward the lower junction; subsequent iteration follows the increasing segment and reaches the right root near 1.3. Hence the roots obtained from x0, x1 and x2 are 1.3, 0.6 and 1.3 respectively.

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