If $\sigma_1$ and $\sigma_3$ are the algebraically largest and smallest principal stresses respectively, the value of the maximum shear stress is

1. $\dfrac{\sigma_1 +\sigma_3}{2} \\$
2. $\dfrac{\sigma_1 -\sigma_3}{2} \\$
3. $\sqrt{\dfrac{\sigma_1 +\sigma_3}{2}} \\$
4. $\sqrt{\dfrac{\sigma_1 -\sigma_3}{2}}$