Compute the integer k>2 for which log10(k−2)!+log10(k−1)!+2=2log10k!.
Solution — click to reveal
We can write the given equation as log10(k−2)!+log10(k−1)!+log10100=log10(k!)2.Then log10[100(k−2)!(k−1)!]=log10(k!)2,so 100(k−2)!(k−1)!=(k!)2. Then 100=(k−2)!(k−1)!k!⋅k!=k(k−1)⋅k=k3−k2.So, k3−k2−100=0, which factors as (k−5)(k4+4k+20)=0. The quadratic factor has no integer roots, so k=5.