Finding the Missing Angle in a Triangle

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Finding the Missing Angle in a Triangle

• Created by Kristin King, Desert Willow
• 12-30-10

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Standard G5.4M.C1.PO1. Draw and label
2-dimensional figures given specific attributes
including angle measure and side
lengths. Essential Questions What is a 2-D
figure? How can it be drawn using a ruler or
protractor? Today I can determine the unknown
measurement of an angle in a triangle.
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Fact All of the angles in any given triangle
add up to 180.º
Dont believe me???
Let me prove it to you!!!
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Step 1
B

• Draw a right triangle.

Label each angle.
A
C
Cut out the triangle.
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Step 2

• Cut off the angles as shown.

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Step 3

• Place the angles together at a point to form a
  straight line.

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The angles placed together form a straight line.
A straight line or straight angle is equal to
180º.
Therefore, the sum of all the angles in a
triangle is equal to 180!º
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You can use this fact to find the missing angle
in a triangle.
For example, lets say I have the following
triangle
I know the measurements of two of the angles, but
the third one is a mystery. How do I find the
measurement of that third angle?
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Remember that all the angles in a triangle added
together equal 180.º
So 80º 45º ? º 180º
Its an equation!
Step 1 Add 80º 45º
125º ?º 180º
Step 2 Subtract 125º from both sides
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Lets try another
32º 24º ?º 180º
Step 1 Add 32º 24º
56º ?º 180º
Step 2 Subtract 56º from both sides
?º 124º
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This one is a bit tricky...but I think you can do
it!!!
When you remember what that square corner means,
put your finger by your nose. (Remember no nose
picking allowed!)
Hmmm....this one has a 45º angle, a mystery
angle, and an angle with a square corner.
Yes! The square corner means that it is a right
angle.
I know that a right angle is equal to 90.º
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So... 45º 90º ?º 180º
Step 1 Add 45º 90º
135º ?º 180º
Step 2 Subtract 135º from both sides
?º 45º
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Try a few on your own!
? 35º
?º 53º
? 36º
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Closure
On your slate write down the steps to finding the
measurement of the missing angle in a triangle.
http//world.std.com/eshu/dbug/Jeopardy_Think_Mus
ic.mp3