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Question

If a, b, c are positive integers, then the co-efficient of x in capital delta equals open vertical bar table row cell 1 plus x end cell cell open parentheses 1 plus x close parentheses to the power of a end exponent end cell cell open parentheses 1 plus x close parentheses to the power of b c end exponent end cell row cell 1 plus x end cell cell open parentheses 1 plus x close parentheses to the power of b end exponent end cell cell open parentheses 1 plus x close parentheses to the power of c a end exponent end cell row cell 1 plus x end cell cell open parentheses 1 plus x close parentheses to the power of c end exponent end cell cell open parentheses 1 plus x close parentheses to the power of a b end exponent end cell end table close vertical bar is

  1. a + b + c    
  2. abc    
  3. a2 + b2 + c2    
  4. 0.    

The correct answer is: a2 + b2 + c2


    Each term in the expansion of ∆ is of the form open parentheses 1 plus x close parentheses to the power of m end exponent, where m is a +ve integer.
    We have open parentheses 1 plus x close parentheses to the power of m end exponent= 1 plus m x plus fraction numerator 1 over denominator 2 end fraction m open parentheses m minus 1 close parentheses x to the power of 2 end exponent plus......
    Co-eff. of x in open parentheses 1 plus x close parentheses to the power of m end exponent= ∆’ (0) = 0
    We have
    capital delta ´ open parentheses x close parentheses equals open vertical bar table row 1 cell open parentheses 1 plus x close parentheses to the power of a end exponent end cell cell open parentheses 1 plus x close parentheses to the power of b c end exponent end cell row 1 cell open parentheses 1 plus x close parentheses to the power of b end exponent end cell cell open parentheses 1 plus x close parentheses to the power of c a end exponent end cell row 1 cell open parentheses 1 plus x close parentheses to the power of c end exponent end cell cell open parentheses 1 plus x close parentheses to the power of a b end exponent end cell end table close vertical bar plus open vertical bar table row cell 1 plus x end cell cell a open parentheses 1 plus x close parentheses to the power of a minus 1 end exponent end cell cell open parentheses 1 plus x close parentheses to the power of b c end exponent end cell row cell 1 plus x end cell cell b open parentheses 1 plus x close parentheses to the power of b minus 1 end exponent end cell cell open parentheses 1 plus x close parentheses to the power of c a end exponent end cell row cell 1 plus x end cell cell c open parentheses 1 plus x close parentheses to the power of c minus 1 end exponent end cell cell open parentheses 1 plus x close parentheses to the power of a b end exponent end cell end table close vertical bar plus open vertical bar table row cell 1 plus x end cell cell open parentheses 1 plus x close parentheses to the power of a end exponent end cell cell b c open parentheses 1 plus x close parentheses to the power of b c minus 1 end exponent end cell row cell 1 plus x end cell cell open parentheses 1 plus x close parentheses to the power of b end exponent end cell cell c a open parentheses 1 plus x close parentheses to the power of c a minus 1 end exponent end cell row cell 1 plus x end cell cell open parentheses 1 plus x close parentheses to the power of c end exponent end cell cell a b open parentheses 1 plus x close parentheses to the power of a b minus 1 end exponent end cell end table close vertical bar
    therefore capital delta ´ open parentheses 0 close parentheses equals open vertical bar table row 1 1 1 row 1 1 1 row 1 1 1 end table close vertical bar plus open vertical bar table row 1 a 1 row 1 b 1 row 1 a 1 end table close vertical bar plus open vertical bar table row 1 1 cell b c end cell row 1 1 cell c a end cell row 1 1 cell a b end cell end table close vertical bar equals 0 plus 0 plus 0 equals 0

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