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Consider a system of equations where the i^th equation is ai Φi=bi Φ(i+1)+ci Φ(i+1)+di. While solving this system using Thomas algorithm, we get Φi=Pi Φ(i+1)+Qi. What are Pi and Qi?(a) \(P_i=\frac{c_i Q_{i-1}+d_i}{a_i-c_i P_{i-1}};Q_i=\frac{b_i}{a_i-c_i P_{i-1}}\)(b) \(P_i=\frac{b_i}{a_i-c_i P_{i-1}};Q_i=\frac{c_i Q_{i-1}+d_i}{a_i-c_i P_{i-1}}\)(c) \(P_i=\frac{c_i Q_{i-1}+b_i}{a_i-c_i P_{i-1}};Q_i=\frac{d_i}{a_i-c_i P_{i-1}}\)(d) \(P_i=\frac{d_i}{a_i-c_i P_{i-1}};Q_i=\frac{c_i Q_{i-1}+b_i}{a_i-c_i P_{i-1}}\)I have been asked this question by my college director while I was bunking the class.The origin of the question is Discretization Aspects in portion Basic Aspects of Discretization, Grid Generation with Appropriate Transformation of Computational Fluid Dynamics

Answer»

Right choice is (B) \(P_i=\frac{b_i}{a_i-c_i P_{i-1}};Q_i=\frac{c_i Q_{i-1}+d_i}{a_i-c_i P_{i-1}}\)

Best explanation: As GIVEN,

Φi = PiΦi+1+Qi

Φi-1 = Pi-1Φi+Qi-1

The i^th equation is,

aiΦi = BI Φi+1 + ciΦi-1 + di

aiΦi = bi Φi+1 + CI(Pi-1Φi + Qi-1) + di

aiΦi – ciPi-1Φi = bi Φi+1+ciQi-1+di

Φi(ai-ci Pi-1) = biΦi+1+ci Qi-1+di

\(\Phi_i = \frac{b_i}{a_i-c_i P_{i-1}}\Phi_{i+1} + \frac{c_i Q_{i-1}+d_i}{a_i-c_i P_{i-1}} \)

Therefore,

\(P_i = \frac{b_i}{a_i-c_i P_{i-1}};Q_i=\frac{C_i Q_{i-1}+d_i}{a_i-c_i P_{i-1}}\).



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