Numerical Work with BIG Circles
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A BIG circle question combines all (or most) of your circle angle formulas into one question.  You may also need to apply other strategies to find all of the missing angles.

Question:

Circle O with tangent and
Find all of the numbered angles.

Solution:
Find the arcs.  Then find the angles in any order that you wish.

Find the measures of the arcs:
7x + 8x + 12x + 9x = 360
36x = 360
x = 10
arc BC = 70°
arc CD = 80°
arc AD = 120°
arc AB = 90°

label these arcs on the diagram

   <1 is an inscribed angle

m<1= 1/2 arc
        = 1/2 (80)
 = 40°

<2 is an inscribed angle

m<2 = 1/2 arc
           = 1/2 (120)
  = 60°

<3 is an inscribed angle

m<3= 1/2 arc
        = 1/2 (70)
 = 35°

<4 is formed inside the circle by two intersecting chords.
m<4 = 1/2 (sum of arcs)
 = 1/2 (90+80)
= 1/2 (170)
= 85°
Also m<6 = 85° since it is a
vertical angle with <4

<5 is formed inside the circle by two intersecting chords
OR <5 and <4 form a straight angle (line)
m<5 = 180 - 85
= 95°
Also m<7 = 95° since it is a
vertical angle with <5

<8 = a "tricky" angle since it does not FIT any of the circle angle formulas.
There are several strategies to arrive at this answer.
Inscribed <BCA is adjacent to <8 and its size is 45°.  This means <8 must be 135º to form a straight <.
OR
Once you know <6, <9, and <10, you can find <8 by remembering that the angles in a quadrilateral add to 360°.

<9 is formed by a tangent and chord

m<9 = 1/2 arc
= 1/2 (150)
= 75°
Did you use the ENTIRE intercepted arc from B to D?

<10 is formed outside by a tangent and secant

m<10 = 1/2(difference of arcs)
= 1/2 (210 - 80)
= 1/2 (130)
= 65°
Were you careful to use ALL of arc BAD?

<11 = a "tricky" angle since it does not FIT any of the circle angle formulas.
<11 is adjacent to <10 and together they form a straight angle (line).
m<11 = 180 - 65
= 115°

 

<12 is formed outside by a tangent and secant

m<12 = 1/2(difference of arcs)
= 1/2 (120 - 80)
= 1/2 (40)
= 20°
 

Also, many of the angles could be found by remembering that there are 180° in a triangle.