Question

Is anyone good with Geometric Proofs?

I need help!
Ill attach the paper

Answer 1

@Hero

Answer 2

First off, you know the principals relating to angles of lines that intersect?

Answer 3

ok

Answer 4

Not sure if you know them or not. I mean this stuff:

Answer 5

I know a little, but looking at the 2nd problem why does it say PC = QB There is no P or Q

Answer 6

Probably a typo. I beat they meant \(AD\cong BE\).

Answer 7

bet...

Answer 8

On the first one, you can use the fact that the intersections of lines must add up to 180 degrees to get a generic measure for the angles at points B abd C. Then use the givens that the inner angles of B and C are the same to show that those outers are using 180-(the same number) to show thart they are the same. I just do not remember the names of the theroems off hand to show that, but you should have a list somewhere in their book or on ylur class web site of theorems you are using.

Answer 9

Something to do with a transversal making supplementary angles...

Answer 10

But yah, on the second one, I would just ask the teacher if it is supposed to be what it is, since those other points do not exist.

Answer 11

thank you

Answer 12

Ah, here, found this:

Linear Pair Postulate If two angles form a linear pair, then the measures of the angles
add up to 180°.

That looks like the one you need to use in the first one. And lets see... Triangle Exterior Angle Postulate could also do it, which says that "The measure of an exterior angle of a triangle is equal to the sum of the measures of the remote interior angles."

Either of those can get you in the correct direction.

I found a PDF with the different postulates and theorems.

http://web.cerritos.edu/dford/SitePages/Math_70_F13/PostulatesandTheorems.pdf

Answer 13

oh great thank you so much!!

Answer 14

This PDF might also help... but the visual queues did not show up =(

http://www.riverview.wednet.edu/parade/teachers/mcclintict/Geometry-1/Geo-PostulatesTheorems-List-2011.pdf

Answer 15

You are so much help

Answer 16

With proofs, it cames down to finding a set of rules, one after the other, that will let you say "This works!" or "This never works!" in the proper order and with the proper name. So I can tell you that when one segment touches a line, the angles on each side of where it touces add up to 180 degrees... but it does not mean much. On the other hand, if I say Linear Pair Postulate, it means exactly that but is using words that a math teacher likes. Hehe.

Answer 17

I'll have to remember that

Answer 18

what did I do wrong? :'( @e.mccormick

Answer 19

They want you to prove they are conguent segments.

Answer 20

ive tried fixing it and I can quite figure it out where I went wrong ? was it because I put dc in cd and ec instead ce

Answer 21

The DC = EC in step B is wrong. You want to prove that int he end. You could say AC = BC in B.

Answer 22

so b) would be ac = bc , cd = ce?

Answer 23

No, you are given AC cong BC, and some mistake... I am betting that becaus it says you need to prove CD cong CE that the mistake is AD cong BE. If the given is:

AC cong BC, AD cong BE then:
AC = BC, AD = BE by Def. of Cong. Segments

Answer 24

so then c is ac + ad = bc + be for addition property

Answer 25

i thought i knew what i was doing then i messed up and know im 100% lost again

Answer 26

OK, well, the addition property is... hmmm... not sure if you need that at all or just the segment addition postulate.

See, AD + DC = AC, which is adding up the segments to make the line. You can do the same with the other side, so BE + EC = BC.

Then you can say by substitutain that:
AD + EC = BC

For that substitution to be valid, EC = DC.

Does that make some sense?

Answer 27

Oh, and on the AD + EC = BC you can make it clearer by saying:

AD + EC = BC Subsitutuion of AD for BE.

Basically you are saying, "If I can subsitute the upper line segment, then I must also be able to substitute the lower line segment, therefore the lower ones are equal and equal means congruent."

Answer 28

okay let me see if i got this right
a) Ac = bc, ad=be
b)ac=bc, ad = be
c)ac +ad = bc +be
d) ad + dc =ac
be + ec = bc
e)ec =dc
f) dc = ce

Answer 29

@e.mccormick

Answer 30

c, as you have it, really does not need to exists.

c) ad + cd = ac, be + ec = bc, segment addition postulate
d) ad + ce = bc, subsitution of ad for be
e) ad + ce = ac, subsitution of ac for bc

Hmmm..... at that point, I know it is saying CD = CE beacuse they are in the same place int he same equation.. so probably by substitution too....

Answer 31

Or perhaos the converse of subsitution.

Answer 32

ARGH! I know what subsitution I meant!

d) ad + ce = bc, subsitution of ad for be
e) ad + ce = ac, subsitution of ac for bc
f) ad + ce = ad + cd, subsitution of ad + cd for ac
g) ce = cd... this is one of the basic addition ones. Do you remember the name of it?

Answer 33

Additive equality.

Answer 34

So then h) CD = ec

Answer 35

Yah, then you do the cd cong ce because of definition of congruent.

Answer 36

I hope that makes more sense now. Even I went a little sideways on that one for a moment. Well... there is usually more than one way to do it, but on this one a subsitution chain is clear. Being clear is the intent of proofs. Clear and detailed baby steps that lead to the answers is basically what a proof is.

Answer 37

Thank hopefully it's all good

Answer 38

And this is the full name of that additive thing...
http://www.mathwords.com/a/additive_property_of_equality.htm

Answer 39

Okay sweet

Answer 40

@e.mccormick My teacher said it was Ac=bc , Dc= ec not ad=be
im still confused

Answer 41

@e.mccormick

Answer 42

i got it!!!

Answer 43

Well, that would not make much differrence, and it seems you were able to apply what was done yesterday to fix it.

Answer 44

A) AC≅BC, DC≅EC
B) AC = CB, DC=CE
C) AC + CB = DC = CE
D) DC + AC = DC
CB + CE =CE
E) DC = CE
F) DC ≅ CE

Answer 45

if that isnt right then i give up on this assignment

Answer 46

Wait.... the teacher said it is DC≅EC at the top in the givens?

Answer 47

yes with ac = bc

Answer 48

Then it is stupidly simple.

A) DC≅EC
B) DC = CD, EC = CE: defenition of a line segment
C) CD≅CE: substitution of CD for DC and CE for EC.


I think your teacher made a mistake, and what we did yesterday is probably right. I do not think they would ask something that simple.

See, what I did here is the line segment from D to C is the same as from C to D based on the definition of a line segment.

Answer 49

okay so other than that part its okay?

Answer 50

No, I am saying that what your teacher gave you as a "correcton" means it can be done in those 3 steps.

If I go from point D to C, it is the same as if I go from C to D. Your teacher is saying that the answer is a given... which makes no sense for a proof.

Answer 51

im so confused

Answer 52

i thought my teacher wanted me to do both not just the dc=ec

Answer 53

Well, what we did yesterday works well... but your teacher is saying that the correction to the typo is for what you want to prove to be is as a given. That just does not make sense.

Answer 54

i dont know ill just figure it out i guess ..

Answer 55

What I am saying is that your teacher seems to have messed up in fixing it. It is like saying, "Prove the the road from New York to New Jersey is the same length as the road from New Jersey to New York."

Your teacher said:

Given AC≅BC, DC≅EC
Prove CD≅CE

Well duh! DC≅EC = CD≅CE


If that is NOT what your teacher meant, then the thing I went over yesterday is the correct answer.

Answer 56

Oh wow im sorry i just fully understood that

ill have to ask her about that

Answer 57

LOL. Yah. I think you got it yesterday and your teacher is still confused. Yesterday we basically did:

a + b = c and x + y = z
Given a = x and c = z, prove that b = y.

That is the short form, and we did all that using the geometric notation for it.

Answer 58

well ill talk to her and let know

Answer 59

let you know *

Answer 60

I am the dyslexic, poor typist here... so a missing word or two is no big deal. =)

Answer 61

haha she said it sounded good i just need the reasons next to those 3

Answer 62

which i got from you thank you

um do you think you can help me with the other problem on that paper?

Answer 63

The top one?

Answer 64

yes

Answer 65

The Linear Pair Postulate I pointed out yesterday, and some substitution because the inner angles are given as equal should do it.

Answer 66

my teacher said the 2 angles are given to be equal, this means it is an isosceles triangle. This should make your proof easier

Answer 67

Well, I do not know of any isosceles triangle specific rules that apply with how I was saying you could do it. I was just pointing out that the inner angles match and can be substituted for each other, which means you can make it so the outer ones must also subsitute... it is actually kind of similar to the second one when we did it the last way we did yesterday.

Answer 68

so A) is m<xbc=m<xbc then for b you would xcb+ xbc=xcd+ xba?

Answer 69

On A, notice the C and B swap positions.
A) m∠XBC = m∠XCB, Given
For B I would do:
m∠XBA = 180 - m∠XBC, and m∠XCD = 180 - m∠XCB: Linear Pair Postulate

Answer 70

what do we do for the next step im starting to get a little confused

Answer 71

A) m∠XBC = m∠XCB, Given

B) m∠XBA = 180 - m∠XBC, and m∠XCD = 180 - m∠XCB: Linear Pair Postulate

Two substitutions and it is done.

With what you know there, what can you subsitute in:
m∠XBA = 180 - m∠XBC

Answer 72

this problem has me completely lost

Answer 73

Perhaps it is all the
m∠XBA, m∠XBC etc stuff. Lets try something to make that simpler.

I will use Bi for the XBC anf Bo for XBA, that is because they are point B's inner and outer angles. Similar things can be done for point C.




Given: Bi = Ci
Prove: Bo = Co

A) Bi = Ci: given

B) Bo = 180 - Bi and
Co = 180 - Ci : Linear Pair Postulate

Do you see where this is going? Two substitutions and it is done.

Answer 74

yes i do understand what you did

Answer 75

@e.mccormick im just having a hard time figuring out what each step like i dont know whats next

Answer 76

i cant wrap my head around it to know what to do next but i do understand whats going on

Answer 77

Well, at this point it is algebra to make them look alike. You just need to do a substitution. See, the line A tells you what substitutions can be done in B.

You were given something. You use a rule to show something. Then you use the given and a new rule to substitute for the next step.

Answer 78

oh okay

Answer 79

Once you do that sub, then you will have one last sunstitution. The result of the first substitution and the other half of B will make for a second subsitution!

Or you could do it a little longer, just putting something on seperate lines:

Given: Bi = Ci
Prove: Bo = Co

A) Bi = Ci: given
B) Bo = 180 - Bi : Linear Pair Postulate
C) Co = 180 - Ci : Linear Pair Postulate
D) subsitution involving lines B) and A)
E) subsitution involving lines C) and D)

And it will be done! That is one operation per line, which is the normal in formal proofs.

Answer 80

okay hold im trying to figure it out lol