How to differentiate amd simplify: ln(cosh(ln x) cos(x)) ?

1 Answer
Mar 9, 2018

dy/dx = tanh(lnx)/x - tanxdydx=tanh(lnx)xtanx

Explanation:

I like to set the problem equal to y if it is not already. Also it will help our case to rewrite the problem using properties of logarithms;

y = ln(cosh(lnx)) + ln(cosx)y=ln(cosh(lnx))+ln(cosx)

Now we do two substitutions to make the problem easier to read;

Let's say w = cosh(lnx)w=cosh(lnx)
and u = cosxu=cosx
now;

y = ln(w) + ln(u)y=ln(w)+ln(u)
ahh, we can work with this :)

Let's take the derivative with respect to x of both sides. (Since none of our variables are x this will be implicit differentiation)

d/dx*y = d/dx*ln(w) + d/dx*ln(u)ddxy=ddxln(w)+ddxln(u)

Well, we know the derivative of lnxlnx to be 1/x1x and using the chain rule we get;

dy/dx = 1/w * (dw)/dx + 1/u*(du)/dxdydx=1wdwdx+1ududx

So let's go back to u and wuandw and find their derivatives

(du)/dx = d/dxcosx = -sinxdudx=ddxcosx=sinx
and
(dw)/dx = d/dxcosh(lnx) = sinh(lnx)*1/xdwdx=ddxcosh(lnx)=sinh(lnx)1x (using the chain rule)

Plugging our newly found derivatives, and u, and w back into dy/dxdydx we get;

dy/dx = 1/cosh(lnx) * sinh(lnx)/x + 1/cosx*-sinxdydx=1cosh(lnx)sinh(lnx)x+1cosxsinx

dy/dx = sinh(lnx)/(xcosh(lnx)) - sinx/cosxdydx=sinh(lnx)xcosh(lnx)sinxcosx

dy/dx = tanh(lnx)/x - tanxdydx=tanh(lnx)xtanx

If this can be simplified further, I haven't learned how. I hope this helped :)