Evaluate: ( 5 + 5·sqrt(3)·i )7
1) Write 5 + 5·sqrt(3)·i in r·cis( theta ) form:
If you have a + bi use r = sqrt(a2 + b2) and theta = tan-1( b/a ):
r = sqrt( ( 5 )2 + ( 5·sqrt(3) )2 = sqrt( 25 + 75 = sqrt( 100 ) = 10
theta = tan-1( 5·sqrt(3) / 5 ) = 60o [If you are to use radians, use pi/3.]
[You need to check which quadrant you should use; since both 5 and 5·sqrt(3) are
positive, the angle is in the first quadrant; so 60o is the correct angle.]
2) You now have: 5 + 5·sqrt(3)·i = 10·cis( 60o )
To raise to a power, raise the constant to that power and multiply the angle by the power.
[ 10·cis( 60o ) ]7 = 107 · cis( 60o · 7 ) = 107 · cis( 420o ) = 107 · cis( 60o )
3) To write this in rectangular form, use: x = r·cos( theta ) and y = r·sin( theta )
x = 107·cos( 60o ) = 107· (1/2) = 5·106
y = 107·sin 60o ) = 107· (sqrt(3)/2) = 5·sqrt(3)·106
---> 5·106 + 5·sqrt(3)·106·i