f'[x] = lim(h->0) (f[x+h]-f[x])/h
lim(h->0) (sin[x+h]-sin[x])/h
By angle-addition formula, we have:
lim(h->0) (sin[x]cos[h]+sin[h]cos[x]-sin[x])/h
lim(h->0) (sin[x]cos[h]-sin[x])/h + lim(h->0) (sin[h]cos[x])/h
sin[x]*lim(h->0) (cos[h]-1)/h + cos[x]*lim(h->0) sin[h]/h
In a calculus class, it is shown that:
lim(h->0) (cos[h]-1)/h = 0 and that lim(h->0) sin[h]/h is 1. So,
sin[x]*lim(h->0) (cos[h]-1)/h + cos[x]*lim(h->0) sin[h]/h becomes
sin[x]*0 + cos[x]*1
cos[x]
So, if f[x] = sin[x], f'[x] = cos[x]
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The derivative of sin (x) is cos (x). It does not work the other way around, though. The derivative of cos (x) is -sin (x).
(cos x sin x) / (cos x sin x) = 1. The derivative of a constant, such as 1, is zero.
Write sec x as a function of sines and cosines (in this case, sec x = 1 / cos x). Then use the division formula to take the first derivative. Take the derivative of the first derivative to get the second derivative. Reminder: the derivative of sin x is cos x; the derivative of cos x is - sin x.
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The derivative of cos x is -sin x, the derivative of square root of x is 1/(2 root(x)). Applying the chain rule, the derivative of cos root(x) is -sin x times 1/(2 root(x)), or - sin x / (2 root x).