text: 1=-1 but how? we know i=sqrt(-1) and i^2=-1 ok watch this. 1 = sqrt(1) = sqrt(-1*-1) =sqrt(-1)*sqrt(-1)=i*i = i^2 = -1 as you see 1=-1 how can this happen? sqrt(1) = sqrt(-1*-1) = sqrt(-1) * sqrt(-1) the last step, sqrt(ab) = sqrt(a)*sqrt(b), only valid for non-negative real numbers a, b.