11. No, need MKJ MKL
12. Yes, by Alt Int Angles SRT UTR and STR URT; RT RT (reflex) so
ΔRST ΔTUR by ASA
13. A D
14. No need to know K and H are rt s
15. Yes, BE CE and AE DE; A D (rt s thm) so ΔRST ΔTUR by HL
20. Proof B incorrect. Corr. sides are not in correct order
24. Since we know 2 sides and included angle, we could use SAS. Since the Δs are rt
s, we could use HL.
38. AB = 6, BC = 8
1. If ∆ABC ∆DEF, then A
? and BC ? .
2. What is the distance between (3, 4) and (–1, 5)?
3. If 1 2, why is a||b?
Converse of Alternate
Interior Angles Theorem
4. List methods used to prove two triangles congruent.
SSS, SAS, ASA, AAS, HL
CPCTC is an abbreviation for the phrase
“Corresponding Parts of Congruent Triangles are
Congruent.” It can be used as a justification in a
proof AFTER you have proven two triangles
SSS, SAS, ASA, AAS, and HL use
corresponding parts to prove triangles
congruent. CPCTC uses congruent
triangles to prove corresponding parts
A landscape architect sets
up the triangles shown in
the figure to find the
distance JK across a pond.
What is JK?
One angle pair is congruent,
because they are vertical
Two pairs of sides are congruent, because their
lengths are equal. Therefore the two triangles are
congruent by SAS. By CPCTC, the third side pair is
congruent, so JK = 41 ft.
Given: YW bisects XZ, XY YZ.
Prove: XYW ZYW
1. YW bisects XZ
2. XW ZW
Def of bisects
3. XY YZ
4. YW YW
5. ΔXYW ΔZYW
6. XYW ZYW
Work backward when planning a proof. To
show that ED || GF, look for a pair of angles
that are congruent.
Then look for triangles that contain these
Given: J is the midpoint of KM and NL.
Prove: KL || MN
1. J is the midpoint of KM and NL. 1. Given
2. KJ MJ, NJ LJ
2. Def. of mdpt.
3. KJL MJN
3. Vert. s Thm.
4. ∆KJL ∆MJN
5. LKJ NMJ
6. Conv. Of Alt. Int. s
6. KL || MN
Example 4: Given: D(–5, –5), E(–3, –1), F(–2, –3),
G(–2, 1), H(0, 5), and I(1, 3)
Prove: DEF GHI
Use the Distance Formula to find the lengths of the sides
of each triangle.
So DE GH, EF HI, and DF GI.
Therefore ∆DEF ∆GHI by SSS, and DEF GHI