Step by step
The question
Question: the line A T touches the circle at A, the angle between it and the chord A B is 35°, and angle A B C is 80°: find angle B A C.
1 Use the rule
The angle between the tangent and the chord A B is 35°.
The alternate segment theorem: it equals the angle at C, on the other side of the chord, in the alternate segment.
So angle A C B is 35°.
angle ACB = 35°
Careful!
Not the angle at B, next to the tangent angle.
The equal angle is across the chord, at C.
Careful: the angle in the OTHER segment, across the chord
2 Use the angle fact
The three angles of triangle A B C add up to 180°.
180 minus 35 minus 80 is 65°.
angle BAC = 180° − 35° − 80° = 65°
3 Check
Let's check: on the straight tangent line at A, the angle on the other side is 80°, by the same rule.
35 plus 65 plus 80 is 180°: a straight line.
So it is right!
on the tangent at A: 35° + 65° + 80° = 180°
Let's look back.
First: use the rule.
Then: use the angle fact.
Last: check.
Answer
angle BAC = 180° − 35° − 80° = 65°