How do you differentiate #sin²(2x)#?

1 Answer
Jul 17, 2015

This is nothing crazy complicated if you get the pattern; you can do the following (I presented this in regular prime notation and function composition notation):

#color(green)("Overall Function")#
#color(purple)("First Inner Function")#
#color(darkblue)("Innermost Function")#
#color(highlight)("Derivative")#

#sin^2(2x) = (sin(2x))^2#

Let:
#color(green)(f{g[h(x)]}) = f[(g @ h)(x)] color(green)(= {color(purple)(g[h(x)])}^2)#
#color(purple)(g[h(x)]) = (g @ h)(x) color(purple)(= sin[color(darkblue)(h(x))])#
#color(darkblue)(h(x) = 2x)#

#d/(dx)(color(green)(f{g[h(x)]})) = color(highlight)(f'){color(purple)(g[h(x)])}*color(highlight)(g')[color(darkblue)(h(x))]*color(highlight)(h'color(black)((x)))#

#= color(highlight)(2)[color(purple)(sin(2x))]*color(highlight)(cos)(color(darkblue)(2x))*color(highlight)(2)#

#= 2(2sin(2x)cos(2x))#

#= color(blue)(2sin(4x))#

...since #sin(2x) = 2sinxcosx# and thus #sin(4x) = 2sin(2x)cos(2x)#.