Question 985805

90°


(because it leans the arc of  180°). 

<TABLE> 
  <TR>
  <TD>
{{{drawing( 250, 250, 0.5, 5.5, -0.5, 4.5, 
            line( 0.764, 2, 5.236, 2 ),
            line( 0.764, 2,    2,     3.98), 
            line( 2,     3.98, 5.236, 2), 

            locate ( 0.6,  2.2,  A),
            locate ( 1.9,  4.3,  B),
            locate ( 5.3,  2.2,  C),

            circle(3.0, 2.0, 0.03, 0.03),
            locate ( 2.96, 1.95, O),

            circle(3.0, 2.0, 2.236, 2.236),

            arc ( 2,  3.98, 0.8, 0.8, 33, 125)
)}}}
&nbsp;&nbsp;<B>Figure</B>. &nbsp;An inscribed angle
leaning on the diameter is a right angle

  </TD>
  </TR>
</TABLE> 

See the lesson <A HREF=http://www.algebra.com/algebra/homework/Circles/An-inscribed-angle.lesson>An inscribed angle in a circle</A> in this site.