suppose $I_r= \int dz/(z(z-1)(z-2))$ along $C_r$, where $C_r = \{z\in\mathbb C : |z|=r\}$, $r>0$. Then
$I_r= 2\pi i$ if $r\in (2,3)$
$I_r= 1/2$ if $r\in (0,1)$
$I_r= -2\pi i$ if $r\in (1,2)$
$I_r= 0$ if $r>3$.
I am stuck on this problem . Can anyone help me please...... I can't solve it with residue theorem......
I don't know where to begin?