20)[tex][6(cos \frac{7 \pi }{8}+isin \frac{7 \pi }{8})]^4=6^4(cos \frac{7*4 \pi }{8}+isin \frac{7*4 \pi }{8})=6^4(cos \frac{7 \pi }{2}+isin \frac{7 \pi }{2})= [/tex]=
[tex] 6^{4}[0+(-1)i]=- 6^{4}i.Exercitiu21) [2(cos20+isin(180-20)]^12=[tex] 2^{12}(cos20*12+isin20*12)= 2^{12}(cos240+isin240)= [/tex][tex] 2^{12}[cos(180+60)+isin(180+60)]= 2^{12}(-cos60-sin60)= [/tex][tex] 2^{12}(- \frac{1}{2}- \frac{ \sqrt{3} }{2}i) [/tex]
