2006-II-41
Ans: C
Sub. $y=0$ into the equation of $AB$, we have
… Read Sub. $y=0$ into the equation of $AB$, we have
$\begin{
$\begin{
Note that each faces of the tetrahedron are equilater
$\begin{array}{ll}
OB=OC & \t
In $\Delta OAB$,
$\begin{array}{ll}
OB = B
$\begin{array}{cl
$\begin{array}{|c|c|c
$\begin{array}{cl}
=