Bằng định nghĩa:
+ [imath]\Delta y = f(x_0+ \Delta x)-f(x_0) \\
= \sin{(2.60^o-30^o+ \Delta x)} - \sin{(2.60^o-30^0)} \\
= \sin{(90^0 +\Delta x)}- \sin{90^o} \\
= \sin{(90^o+ \Delta x)} - 1[/imath]
+ [imath]\dfrac{ \Delta y}{ \Delta x} = \dfrac{\sin{(90^o+ \Delta x)} - 1}{\Delta x}[/imath]
+ [imath]y'=\lim_{ \Delta x \rightarrow 0}{\dfrac{ \Delta y}{ \Delta x}}= \lim_{ \Delta x \rightarrow 0}{\dfrac{\sin{(90^o+ \Delta x)} - 1}{\Delta x}}=0[/imath]
Bằng công thức:
[imath]y= \sin{(2x-30^o)}[/imath]
[imath]y'= (2x-30^o)' \cos{(2x-30^o)} \\
= 2 \cos{(2x-30^o)} \\
=2 \cos{(2.60^o-30^o)} \\
= 2 \cos{90^o}=0[/imath]