Question & Answer
QUESTION

If AB is a chord of a circle with centre O, AOC is a diameter and AT is the tangent at A as shown in figure. Prove that \[\angle BAT = \angle ACB\] .

ANSWER Verified Verified
Hint: In this question, we use the properties of circle and tangent at any point on the circle. Like we use properties, diameter of a circle subtends a right angle to any point on the circle and tangent to a circle is always perpendicular to the radius.

Complete step-by-step answer:

Given that AC is a diameter and AT is the tangent at A as shown in figure.
Let $\angle BAT = \theta $
Now, we know tangent to a circle is always perpendicular to the radius.
Angle between the tangent AT pass through point A and radius OA is 900.
$
   \Rightarrow \angle OAT = {90^0} \\
   \Rightarrow \angle OAB + \angle BAT = {90^0} \\
$
We know, $\angle BAT = \theta $
$
   \Rightarrow \angle OAB + \theta = {90^0} \\
   \Rightarrow \angle OAB = {90^0} - \theta \\
 $
Now, In $\vartriangle ABC$
$\angle ABC = {90^0}$ (Diameter of a circle subtends a right angle to any point on the circle)
 Sum of all angles of $\vartriangle ABC$ is ${180^0}$
$ \Rightarrow \angle CAB + \angle ABC + \angle ACB = {180^0}$
As we know, $\angle ABC = {90^0}$ and $\angle OAB = \angle CAB = {90^0} - \theta $
$
   \Rightarrow {90^0} - \theta + {90^0} + \angle ACB = {180^0} \\
   \Rightarrow {180^0} - \theta + \angle ACB = {180^0} \\
   \Rightarrow \angle ACB = \theta \\
   \Rightarrow \angle ACB = \angle BAT = \theta \\
$
Hence proved, \[\angle BAT = \angle ACB\]

Note: In such types of problems we have to prove LHS (Left hand side) equal to RHS (Right hand side) so we assume LHS angle is $\theta $ and then find the angle of triangle $\vartriangle ABC$ by using the properties of circle and tangent. Then find the RHS angle by using the sum of all angles of triangles equal to ${180^0}$. So, we proved that LHS=RHS.