Differentiate each of the following functions from the first principal :
log cos x
We have to find the derivative of log cosx with the first principle method, so,
f(x) = log cos x
by using the first principle formula, we get,
f ‘(x) = 
f ‘(x) = 
f ‘(x) = 
f ‘(x) = 
[Adding and subtracting 1]
f ‘(x) = 
[Rationalising]
f ‘(x) = 
[By using limx→0 
= 1]
f ‘(x) = 
[cosC – cosD = –2 sin
sin
]
f ‘(x) = 
[By using limx→0 
= 1]
f ‘(x) = 
f ‘(x) = – tan x
6