# Re: use a calculator to find all solutions-standard form

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Posted by cavs on September 25, 2002 at 15:59:46:

In Reply to: Re: use a calculator to find all solutions-standard form posted by T.Gracken on September 25, 2002 at 13:09:35:

: : Need help!!!!!!!
: : x^3 + 4 - 5i =0

: : x^3=-4 + 5i
: : I was able to change it to trigonometric form and got the reference angle of 128.70

: : The radius I believe is ^3sqrt41?????? =3.44821724=3.45

: I believe your problem may lie here. the "radius" is sqrt(41). [that is the absolute value of (-4+5i)]. Now use this result when using the root theorem (corrollary of DeMoivre's Thm.). so you should get "cube root of (sqrt(41))" or appx. 1.857 instead of 3.45

: if I use the calculator and hit the cube root of sqrt button then (41), I get apprx. 3.45. I do not understand how 1.857 came about. 1.857 is the right answer but i am still list
: : I use the formula a= x/n + 360(k)/n

: : I got 42.9 + 120(0)=42.9
: : 42.9 + 120(1)=162.90
: : 42.9 + 120(2)= 282.9
: : so you should get:

: : 3.45(cis 42.9)
: : 3.45(cis 162.90)
: : 3.34(cis 282.90)

: so you should now have

:
: 1.857(cis 42.9)
: 1.857(cis 162.90)
: 1.857(cis 282.90)

:

: :
: : Looks good but I kept getting the wrong answer. What is wrong!!!

: :

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