The angle in a semi-circle is a right angle.[br][br]In plain terms:[br][br]The angle at the centre - in this case the diameter - is [math]180^\circ[/math][br]Just as in Circle Theorem 2, the angle at the centre is twice that as at the circumference. So, if the angle at the centre is [math]180^\circ[/math] then the angle at any point on the circumference will be half of that: [math]90^\circ[/math]