Permutations 6450

Seven times the permutations of n elements equal one-eighth of the permutations of n + 2 elements. What is the number of elements?

Correct answer:

n =  6

Step-by-step explanation:

=(n+2) / =(n+2)(n+1)n /  8 7=(n+2)(n+1)  8 7=(n+2)(n+1) n23n+54=0 n2+3n54=0  a=1;b=3;c=54 D=b24ac=3241(54)=225 D>0  n1,2=2ab±D=23±225 n1,2=23±15 n1,2=1.5±7.5 n1=6 n2=9  n>0 n=n1=6

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