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The number of distinct real roots of open vertical bar table row cell sin invisible function application x end cell cell cos invisible function application x end cell cell cos invisible function application x end cell row cell cos invisible function application x end cell cell sin invisible function application x end cell cell cos invisible function application x end cell row cell cos invisible function application x end cell cell cos invisible function application x end cell cell sin invisible function application x end cell end table close vertical bar equals 0 in the interval – fraction numerator pi over denominator 4 end fraction less or equal than x less or equal than fraction numerator pi over denominator 4 end fraction is

  1. 0  
  2. 2  
  3. 1  
  4. 3  

The correct answer is: 1


    To simplify the determinant, let sin invisible function application x equals a semicolon cos invisible function application x equals b. Then the equation becomes
    open vertical bar table row a b b row b a b row b b a end table close vertical bar equals 0. Operating C subscript 2 end subscript rightwards arrow C subscript 2 end subscript minus C subscript 1 end subscript semicolon C subscript 3 end subscript rightwards arrow C subscript 3 end subscript minus C subscript 2 end subscript, we get
    open vertical bar table row a cell b minus a end cell 0 row b cell a minus b end cell cell b minus a end cell row b 0 cell a minus b end cell end table close vertical bar equals 0
    rightwards double arrow a open parentheses a minus b close parentheses to the power of 2 end exponent minus open parentheses b minus a close parentheses open square brackets b open parentheses a minus b close parentheses minus b open parentheses b minus a close parentheses close square brackets equals 0
    rightwards double arrow a open parentheses a minus b close parentheses to the power of 2 end exponent minus 2 b blank open parentheses b minus a close parentheses open parentheses a minus b close parentheses equals 0
    rightwards double arrow open parentheses a minus b close parentheses to the power of 2 end exponent open parentheses a minus 2 b close parentheses equals 0
    rightwards double arrow a equals b or a equals 2 b
    rightwards double arrow fraction numerator a over denominator b end fraction equals 1 or fraction numerator a over denominator b end fraction equals 2
    rightwards double arrow tan invisible function application x equals 1 or tan invisible function application x equals 2
    But we have – pi divided by 4 less or equal than x less or equal than pi divided by 4
    rightwards double arrow tan invisible function application left parenthesis pi divided by 4 right parenthesis less or equal than tan invisible function application x less or equal than tan invisible function application left parenthesis pi divided by 4 right parenthesis
    rightwards double arrow negative 1 less or equal than tan invisible function application x less or equal than 1
    therefore tan invisible function application x equals 1 blank rightwards double arrow x equals pi divided by 4
    Therefore, there is only one real root

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