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Question

Let three matrices A = open square brackets table row 2 1 row 4 1 end table close square brackets; B = open square brackets table row 3 4 row 2 3 end table close square bracketsand C = open square brackets table row 3 cell – 4 end cell row cell – 2 end cell 3 end table close square brackets then tr(A) + tropen parentheses fraction numerator A B C over denominator 2 end fraction close parentheses + tropen parentheses fraction numerator A left parenthesis B C right parenthesis to the power of 2 end exponent over denominator 4 end fraction close parentheses + tropen parentheses fraction numerator A left parenthesis B C right parenthesis to the power of 3 end exponent over denominator 8 end fraction close parentheses+ ....... + straight infinity =

  1. 6    
  2. 9    
  3. 12    
  4. none    

hintHint:

Trace of matrix is sum of its diagonal elements from the upper left to lower right, of matrix.
Multiply the B and C using matrix multiplication and Find the matrix BC and we observe the Geometric progression with common ratio  using sum upto infinite in Gp solve the value. .

The correct answer is: 6


    tr(A)  = trace of open square brackets table row 2 1 row 4 1 end table close square brackets = 2+1 = 3
    B C space equals space open square brackets table row 3 4 row 2 3 end table close square brackets space open square brackets table row 3 cell negative 4 end cell row cell negative 2 end cell 3 end table close square brackets space equals open square brackets table row cell 9 minus 8 end cell cell space space minus 12 plus 12 end cell row cell 6 minus 6 end cell cell negative 8 plus 9 end cell end table close square brackets
equals open square brackets table row 1 0 row 0 1 end table close square brackets space equals text I (unit matrix) end text
    then tr(A) + tropen parentheses fraction numerator A B C over denominator 2 end fraction close parentheses + tropen parentheses fraction numerator A left parenthesis B C right parenthesis to the power of 2 end exponent over denominator 4 end fraction close parentheses + tropen parentheses fraction numerator A left parenthesis B C right parenthesis to the power of 3 end exponent over denominator 8 end fraction close parentheses+ ....... + straight infinity
    tr(A) + tropen parentheses fraction numerator A text  I end text over denominator 2 end fraction close parentheses + tropen parentheses fraction numerator A left parenthesis text I end text right parenthesis squared over denominator 4 end fraction close parentheses + tropen parentheses fraction numerator A left parenthesis text I end text right parenthesis cubed over denominator 8 end fraction close parentheses+ ....... + straight infinity = tr(A) + tropen parentheses A over 2 close parentheses + tropen parentheses A over 4 close parentheses + tropen parentheses A over 8 close parentheses+ ....... + straight infinity
    As we know t subscript r left parenthesis A over k right parenthesis space equals space 1 over k space t subscript r left parenthesis A right parenthesis then
    tr(A) + tropen parentheses fraction numerator A B C over denominator 2 end fraction close parentheses + tropen parentheses fraction numerator A left parenthesis B C right parenthesis to the power of 2 end exponent over denominator 4 end fraction close parentheses + tropen parentheses fraction numerator A left parenthesis B C right parenthesis to the power of 3 end exponent over denominator 8 end fraction close parentheses+ ....... + straight infinity  =  tr(A) + 1 halftr(A)+ 1 fourthtr(A) + 1 over 8tr(A)+ ....... + straight infinity
    tr(A)( 1 + 1 half1 fourth + 1 over 8+ ....... + straight infinity)
    Here, common ratio  r is 1 half (< 1) and a = 1 the sum upto infinite = fraction numerator a over denominator 1 minus r end fraction
    = 3 left parenthesis fraction numerator 1 over denominator 1 minus begin display style 1 half end style end fraction right parenthesis space equals space 3 space cross times fraction numerator 1 over denominator space left parenthesis begin display style 1 half end style right parenthesis space end fraction space equals space 3 cross times space 2 space equals 6
    Therefore, the value of tr(A) + tropen parentheses fraction numerator A B C over denominator 2 end fraction close parentheses + tropen parentheses fraction numerator A left parenthesis B C right parenthesis to the power of 2 end exponent over denominator 4 end fraction close parentheses + tropen parentheses fraction numerator A left parenthesis B C right parenthesis to the power of 3 end exponent over denominator 8 end fraction close parentheses+ ....... + straight infinity = 6.

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