Transcript Finding the Missing Angle in a Triangle

Finding the Missing
Angle in a Triangle
Created by Kristin King, Desert Willow
12-30-10
Standard:
G5.4M.C1.PO1. Draw and label 2dimensional 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.
Fact: All of the angles in any
given triangle add up to 180.º
Don’t believe me???
Let me prove it to you!!!
Step 1
B
Draw a right triangle.
Label each angle.
Cut out the triangle.
A
C
B
Step 2
Cut off the angles as shown.
A
C
Step 3
Place the angles together at a
point to form a straight line.
B
A
C
B
A
C
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
ou can use this fact to find the missing angle in a triangle
For example, lets say I have the following triangle:
?
80º
45º
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?
Remember that all the angles in a triangle added
together equal 180.º
So: 80º + 45º + ? º = 180º
?
It’s an equation!
80º
45º
Step 1: Add 80º + 45º
125º + ?º = 180º
Step 2: Subtract 125º from both sides
?º = 55º
√Check: 80º + 45º +
Answer!
55º = 180º
Lets try another:
24º
?º
32º
32º + 24º + ?º = 180º
Step 1: Add 32º + 24º
56º + ?º = 180º
Step 2: Subtract 56º from both sides
?º = 124º
Answer!
√Check: 32º + 24º + 124º = 180º
This one is a bit tricky...but I think you can do it!!!
Hmmm....this one has 45º
a 45º angle, a mystery
angle, and an angle
with a square corner.
When you remember what
that square corner means,
put your finger by your
nose.
(Remember: no nose
?º
picking allowed!)
Yes! The square corner means that it is a right angle.
I know that a right angle is
equal to 90.º
45º
?º
So... 45º + 90º + ?º = 180º
Step 1: Add 45º + 90º
135º + ?º = 180º
Step 2: Subtract 135º from both sides
?º = 45º
Answer!
√ Check: 45 + 90º + 45º = 180º
Try a few on your own!
55º
75º
52º
? = 35º
?º
?º
?º = 53º
33º
111º
? = 36º
?º
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_M
usic.mp3