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find the values of r

r^4 − 2r^3 +r^2 + 8r − 20=0

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Arthur D. | Effective Mathematics TutorEffective Mathematics Tutor
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r^4-2r^3+r^2+8r-20=0
using synthetic division:
 
            1     -2     1     8     -20
 
2                  2      0     2     20
 
            1      0     1    10      0
(x-2)
 
 
            1      0     1    10
 
-2                -2    4   -10
 
           1      -2    5     0
(x+2) 
 
using x^2-2x+5=0
x^2-2x=-5
x^2-2x+1=-5+1
x^2-2x+1=-4
(x-1)^2=-4
(x-1)=+√(-4)  
x=1+2i  where i=√(-1)
x=2
x=-2