Expand using DeMoivre's formula? (2sqrt3 -2i)^5

1 Answer
Dec 9, 2017

#(2sqrt3 -2i)^5=-512sqrt3-i512#

Explanation:

#z=2sqrt3 -2i#

#|z|=sqrt((2sqrt3)^2 +(2)^2)=sqrt(4*3+4)=4#

DeMoivre's formula:
#[|z|(cosvarphi+isinvarphi)]^n=|z|^n(cos(nvarphi)+isin(nvarphi))#

Let: #a=2sqrt3# and #b=-2#
In order to find #varphi# the following statement must be true:
#cosvarphi=a/|z|# #^^# #sinvarphi=b/|z|#

#cosvarphi=(2sqrt3)/4=sqrt3/2 =>#

#=> varphi_1=pi/6# #vv# #varphi_2=(11pi)/6#

#sinvarphi=-2/4=-1/2 =>#

#=> varphi_1=(5pi)/6# #vv# #varphi_2=(11pi)/6#

The right #varphi# is the one that matches which is #varphi=(11pi)/6#

YOU CAN SIMPLY SKIP THIS BY WRITING A CIRCLE WITH RADIUS EQUAL TO 1 AND POINTING TREIR VALUES AND FINDING COMMON ANGLE

#(2sqrt3 -2i)^5=#

#=[|4|(cos((11pi)/6)+isin((11pi)/6))]^5=#

#=4^5(cos(5(11pi)/6)+isin(5(11pi)/6))=#

#=1024(cos((55pi)/6)+isin((55pi)/6))=#

#55pi=12*2*2pi+7pi#

#=1024(cos((7pi)/6)+isin((7pi)/6))=#

#=1024(-cos(pi/6)+isin(-pi/6))=#

#=1024(-sqrt3/2-i1/2)=-512sqrt3-i512#

(Hi guys, do you use the same process?)