Mark M. answered 10/10/24
Retired math prof. Calc 1, 2 and AP Calculus tutoring experience.
If the function is y = xcos(3x), then
lny = ln(xcos(3x)) = cos(3x)(lnx)
So, y'/y = -3sin(3x)(lnx) + cos(3x)/x
y' = y[-3sin(3x) lnx + cos(3x)/x] = xcos(3x) [-3sin(3x) + cos(3x)/x]
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On the other hand, if the function is y = xcos^3(x), then
lny = cos3x(lnx)
So, y'/y = -3cos2xsinx lnx + cos3x/x
y' = y[-3cos2xsinx lnx + cos3x/x] = xcos^3(x)(cos2x)(-3sinxlnx + cosx/x)
Bradford T.
I think it is cos^3(x) and not cos(3x) which changes things10/10/24