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Calculus

GATE CSE 2014 Set 2 · Q46

MCQ2 marksMedium

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In the Newton-Raphson method, an initial guess of x0=2x_0 = 2 is made and the sequence x0,x1,x2…x_0, x_1, x_2 \dots is obtained for the function 0.75x3−2x2−2x+4=00.75x^3 - 2x^2 - 2x + 4 = 0 Consider the statements (I) x3=0x_3 = 0. (II) The method converges to a solution in a finite number of iterations. Which of the following is TRUE?

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Solution

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Official answer: A) Only I

Calculus

Let f(x)=0.75x3−2x2−2x+4=34x3−2x2−2x+4f(x) = 0.75x^3 - 2x^2 - 2x + 4 = \frac{3}{4}x^3 - 2x^2 - 2x + 4.

The derivative is:

f′(x)=2.25x2−4x−2=94x2−4x−2f^{\prime}(x) = 2.25x^2 - 4x - 2 = \frac{9}{4}x^2 - 4x - 2

The Newton-Raphson iteration formula is:

xn+1=xn−f(xn)f′(xn)x_{n+1} = x_n - \frac{f(x_n)}{f^{\prime}(x_n)}

Iteration 1: For x0=2x_0 = 2:

f(2)=0.75(8)−2(4)−2(2)+4=6−8−4+4=−2f(2) = 0.75(8) - 2(4) - 2(2) + 4 = 6 - 8 - 4 + 4 = -2
f′(2)=2.25(4)−4(2)−2=9−8−2=−1f^{\prime}(2) = 2.25(4) - 4(2) - 2 = 9 - 8 - 2 = -1
x1=2−−2−1=2−2=0x_1 = 2 - \frac{-2}{-1} = 2 - 2 = 0

Iteration 2: For x1=0x_1 = 0:

f(0)=4f(0) = 4
f′(0)=−2f^{\prime}(0) = -2
x2=0−4−2=0−(−2)=2x_2 = 0 - \frac{4}{-2} = 0 - (-2) = 2

Iteration 3: Since x2=x0=2x_2 = x_0 = 2, we get:

x3=x1=0x_3 = x_1 = 0

Thus, statement (I) is TRUE as x3=0x_3 = 0.

Statement (II) is FALSE because the sequence oscillates endlessly between 22 and 00 (2,0,2,0,…2, 0, 2, 0, \dots) and never converges to a root.

Therefore, only statement I is true.

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This Calculus previous year question is Q46 of the GATE CSE 2014 Set 2 paper. Find more GATE CSE PYQs on Calculus in Engineering Mathematics, or browse all GATE CSE PYQs.

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GateAI solution cardGATE CSE 2014 Set 2 · Q46 · MCQ

In the Newton-Raphson method, an initial guess of x0 = 2 is made and the sequence x0, x1, x2 is obtained for the function 0.75x3 - 2x2 - 2x + 4 = 0 Consider the statements (I) x3 = 0. (II) The method converges to a solution in a finite number of iterations. Which of the following is TRUE?

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Independent practice explanation verified by the GateAI team. GATE is conducted by the IITs; question text follows the official paper.