tanh(x)=sinh(x)/cosh(x)

[sinh(x)]'=cosh(x)

[cosh(x)]'=sinh(x)

[tanh(x)]'=[cosh²(x)-sinh²(x)]/cosh²(x)      =1-tanh²(x)

[tanh(x)]''=-2tanh(x)[tanh(x)]' =-2tanh(x)[1-tanh²(x)]       =2tanh³(x)-2tanh(x)