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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° |
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<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? |
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<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°
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<12 is formed outside
by a tangent and secant
m<12
= 1/2(difference of arcs)
= 1/2 (120 - 80)
= 1/2 (40)
= 20°
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