If x+1/x=4,find the value of (x-1/x)^2
\(\left(x-\dfrac{1}{x}\right)^2=\\ x^2-2+\dfrac{1}{x^2}=\\ x^2+2+\dfrac{1}{x^2}-4=\\ \left(x+\dfrac{1}{x}\right)^2-4=\\ 4^2-4=16-4=12\)
