Question

Linear Algebra,

So I have to find this system with a
elimination,
http://screencast.com/t/beKqY9YJjo

And I got the solution here but I don't understand it:


What did he subtract exactly ? http://screencast.com/t/JtHpAYV7jWL

Thanks a lot

Answer 1

(x+3y =-4) - ( x+3y = -5)
---> ( x+3y-x-3y = -5 - (-4) )

Answer 2

subtracting 2 equations means subtracting their corresponding left and right sides..

Answer 3

- ( x+3y = -5) = -x -ey = 5 right ?

Answer 4

-x -3y = 5

Answer 5

yes

Answer 6

then I have 0 = 1 and not 0 = -1

Answer 7

solution says 0 = -1

Answer 8

if you do

(x+3y =-4) - ( x+3y = -5)
you get 0=1

if you do
-(x+3y =-4) + ( x+3y = -5)
which actually he also did
you get 0=-1

Answer 9

ok and what about the very first equation ? Why I don't use it to solve my problem ?

Answer 10

he said "subtracting 1st from 3rd"
-(x+3y =-4) + ( x+3y = -5)

what ? we used 1st equation, right ?

Answer 11

uuuhhh sorry I meant the 2nd equation

Answer 12

thats because we could figure out the consistency using 1st and 3rd...
so whatever maybe the 2nd equation
the system has to remain inconsistent

Answer 13

Ok I got it now, thanks

Answer 14

welcome ^_^