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Easy

Question

The coefficient of x to the power of 3 end exponent y to the power of 4 end exponent z to the power of 5 end exponent in the expansion of left parenthesis x y plus y z plus x z right parenthesis to the power of 6 end exponent is

  1. 70    
  2. 60    
  3. 50    
  4. None of these    

The correct answer is: 60


    We have,
    left parenthesis x y plus y z plus z x right parenthesis to the power of 6 end exponent equals not stretchy sum for r plus s plus t equals 6 of fraction numerator 6 factorial over denominator r factorial s factorial t factorial end fraction open parentheses x y close parentheses to the power of r end exponent open parentheses y z close parentheses to the power of s end exponent open parentheses z x close parentheses to the power of t end exponent
    equals not stretchy sum for r plus s plus t equals 6 of fraction numerator 6 factorial over denominator r factorial s factorial t factorial end fraction x to the power of r plus t end exponent y to the power of r plus s end exponent z to the power of s plus t end exponent
    If the general term in the above expansion contains x to the power of 3 end exponent y to the power of 4 end exponent z to the power of 5 end exponent comma then
    r plus t equals 3 comma blank r plus s equals 4 and s plus t equals 5
    Also, r plus s plus t equals 6
    On solving these equations, we get
    r equals 1 comma blank s equals 3 comma blank t equals 2
    therefore C o e f f i c i e n t blank o f blank x to the power of 3 end exponent y to the power of 4 end exponent z to the power of 5 end exponent equals fraction numerator 6 factorial over denominator 1 factorial 3 factorial 2 factorial end fraction equals 60

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