Question

Angie did the following proof in her logic class. Which step in the indirect proof did she do incorrectly?
Prove: 8 is divisible by 4.

Step 1: Assume that 8 is not divisible by four.

Step 2: 8 is divisible by 4

Step 3: 8 is divisible by 4.

Answer 1

can u help? @perl

Answer 2

step 2

Answer 3

you sure? @sourwing

Answer 4

when we do indirect reasoning, we assume the negation of what we are trying to prove. so step 1 is correct. step 2 doesn't make sense (we dont assume the negation and the opposite of the negation)

Answer 5

ok so its step 2 right?

Answer 6

i think so

Answer 7

can u help with a few more?

Answer 8

@perl

Answer 9

ok

Answer 10

@perl

Answer 11

@timo86m im here

Answer 12

i somehow landed here

Answer 13

number 3 ?

Answer 14

both please

Answer 15

step 3 I think because it is a repeat

Answer 16

no isn't it step 2?

Answer 17

the proof is a bit odd actually

Answer 18

steps 2 and 3 are repeats , not sure why

Answer 19

:( typo

Answer 20

i learned that u state the opposite, then u reason logically, then u conclude the assumption is false

Answer 21

ok i think timo is right

Answer 22

steps 1,2 taken together or steps 2,3 taken together are both wrong

Answer 23

so which one is wrong? step 2 or 3?

Answer 24

can you upload that question

Answer 25

yes one second

Answer 26

yeah im going to go with step 2

Answer 27

how did she go from assuming 8 is not divisible by 4 , to 8 is divisible by 4 ?

Answer 28

ok any idea on what the answer to question 3 is?

Answer 29

we want to prove
{p1 & p2 & p3 & p4 & p5} -> conclusion

Answer 30

?

Answer 31

okay thanks! do u no anybody that can come and double check the answers?

Answer 32

@perl

Answer 33

we want to prove the conditional:
{p1 & p2 & p3 & p4 & p5} -> conclusion

Now if we 'assume' ~c is true, then at least one of the five statements p1, p2, p3, p4 , p5 must be false. Otherwise if they are all true, then it is possible for all the premises to be true while the conclusion is false (since c is false whenever ~c is true),
in which case the original conditional is false. But we are given that the original conditional is true. This forces us to say , at least one of p1,p2,p3,p4,p5 is false . so at most 4 are true

Answer 34

what were u typing haha @perl

Answer 35

oh i was correcting the logic of it

Answer 36

writing logically correct statements is tricky

Answer 37

do u think u can solve one more for me? @perl

Answer 38

ok

Answer 39

@perl

Answer 40

i agree, the logical progression of the statements is inaccurate

Answer 41

my answer is correct?

Answer 42

i think so

Answer 43

ok thanks!