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Permutations And Combinations

Question
CBSEENMA11014195
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Determine n if:

straight C presuperscript 2 straight n end presuperscript subscript 3 colon straight C presuperscript straight n subscript 3 space equals space 12 colon 1

Solution
straight C presuperscript 2 straight n end presuperscript subscript 3 space colon space straight C presuperscript straight n subscript 3 space equals space 12 colon 1
rightwards double arrow              fraction numerator 2 straight n factorial over denominator left parenthesis 2 straight n minus 3 right parenthesis factorial space 3 factorial end fraction colon fraction numerator straight n factorial over denominator left parenthesis straight n minus 3 right parenthesis factorial space 3 factorial end fraction space equals space 12 colon 1

rightwards double arrow   fraction numerator 2 straight n left parenthesis 2 straight n minus 1 right parenthesis space left parenthesis 2 straight n minus 2 right parenthesis space left parenthesis 2 straight n minus 3 right parenthesis factorial over denominator left parenthesis 2 straight n minus 3 right parenthesis factorial space 3 factorial end fraction cross times fraction numerator left parenthesis straight n minus 3 right parenthesis factorial space cross times space 3 factorial over denominator straight n left parenthesis straight n minus 1 right parenthesis space left parenthesis straight n minus 2 right parenthesis space left parenthesis straight n minus 3 right parenthesis factorial end fraction space equals space 12 over 1
rightwards double arrow                            fraction numerator 2 straight n left parenthesis 2 straight n minus 1 right parenthesis left parenthesis 2 straight n minus 2 right parenthesis over denominator straight n left parenthesis straight n minus 1 right parenthesis left parenthesis straight n minus 2 right parenthesis end fraction equals 12 
rightwards double arrow                                     fraction numerator 4 left parenthesis 2 straight n minus 1 right parenthesis over denominator left parenthesis straight n minus 2 right parenthesis end fraction space equals space 12
rightwards double arrow                                        8n - 4 = 12n - 24
rightwards double arrow                                             4n = 20
rightwards double arrow                                               n = 5
 

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