Consider the following stress tensor:

$([300,450],[450,-200])MPa$

Determine a unit vector defining a principal direction.

$0\mathrm{.}862i+0\mathrm{.}507j$

$-0\mathrm{.}862i+0\mathrm{.}507j$

$0\mathrm{.}507i+0\mathrm{.}861j$

$0\mathrm{.}862i-0\mathrm{.}507i$