cosh2(x)-sinh2(x)=1, so, dividing through by cosh2(x):
1-tanh2(x)=sech2(x) is the relationship between sech(x) and tanh(x).
Therefore tanh2(x)=1-sech2(x), and tanh(x)=√(1-sech2(x)).
Alternatively sech(x)=√(1-tanh2(x)).
u=tanh(x)/(1+sech(x))=sinh(x)/[cosh(x)(1+sech(x))=sinh(x)/(cosh(x)+1).