A line that touches a circle at only one point is known as a tangent to the circle.
The common point to the tangent and the circle is known as the point of contact.
Alternate segment theorem
If a line touches a circle and from the point of contact a chord is drawn, then the angles that this chord makes with the given line are equal to the angles formed in the corresponding alternate segments, respectively.
Given: Let AB be a chord of a circle with centre O. PQ be a tangent to the circle at A.
Let E and F be any two points on the circle such that they are in alternate segments R2 and R1, respectively.
To prove: (i) m BAQ = m AEB
(ii) m BAP = m AFB
AEB is an inscribed angle in arc AEB, while arc AFB is the intercepted arc.
m AEB = (1/2)m( arc AFB) (By inscribed angle theorem) ….. (1)
PQ is tangent at A and line AB is a secant.
BAQ intercepts arc AFB
m BAQ = (1/2)m( arc AFB) (By tangent secant theorem) ….. (2)
From (1) and (2)
m AEB = m BAQ
Similarly, m AFB = m BAP.
Converse of Alternate Segment Theorem
If a line is drawn through an end point of a chord of a circle so that the angle formed with the chord is equal to the angle subtended by the chord in the alternate segment, then the line is a tangent to the circle.
Given: AB is a chord of a circle with centre O.
Line PAQ is drawn through A such that m BAQ = m ACB , where C is a point on the circumference.
To prove : PAQ is a tangent to the circle at point A.
Let us suppose that PAQ is not a tangent.
Draw a tangent P'AQ' to the circle at A.
m BAQ' = m ACB (By alternate segment theorem)
But m BAQ = m ACB (Given)
m BAQ = m BAQ'
Unless ray AQ' coincides with AQ, this is impossible.
Therefore, P'AQ' coincides with PAQ.
Or PAQ is a tangent to the circle at A.