Answer:
=ei x
Explanation:
Consider the function on the right hand side (RHS)
f(x) = cos( x ) + i sin( x )
Differentiate this function
f ' (x) = -sin( x ) + i cos( x) = i f(x)
So, this function has the property that its derivative is i times the original function.
What other type of function has this property?
A function g(x) will have this property if
dg / dx = i g
This is a differential equation that can be solved with seperation of variables
(1/g) dg = i dx
integral (1/g) dg = integral i dx
ln| g | = i x + C
| g | = ei x + C = eC ei x
| g | = C2 ei x
g = C3 ei x
So we need to determine what value (if any) of the constant C3 makes g(x) = f(x).
If we set x=0 and evaluate f(x) and g(x), we get
f(x) = cos( 0 ) + i sin( 0 ) = 1
g(x) = C3 ei 0 = C3
These functions are equal when C3 = 1.
Therefore,
cos( x ) + i sin( x ) = ei x