Respuesta :

1/r + 2/1-r = 4/r^2

1-r+2r/r(1-r)=4/r^2
(1+r)/r(1-r)=4/r^2   cancle r both side

1+r/1-r=4/r

cross multiply 
r+r^2=4-4r
r^2+4r+r-4=0
r^2+5r-4=0
r^2+4r+r-4=0
solve it for r factor it...

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