If (a + b), 2b, (b + c) are in HP, then which one of the following is correct?

Let t1, t2, t3 ... be in GP. What is $\rm \left(t_1 t_3 \ldots t_{21}\right)^{\frac{1}{11}}$ equal to ?

Which one of the following is a square root of $-\sqrt{-1} $?

What is the maximum number of points of intersection of 10 circles?