Please help.. Simultaneous problem

[dosftw]

Hi there first post!

I have been doing some maths revision for my maths exam resit on monday and have been brushing up on Simultaneous Equations. I have been using the Maths Watch video's now I have found this program extremely useful until now however during the last topic, it seems to contradict its self.

the example was:

6c - 2d = 33 - - - 1
4c + 3d = 9 - - - 2

1 = x4
2 = x6

which then becomes:

24c - 8d - 132
24c + 18d = 54

therfore:

-26d = 78
d = 78 / -26
d = -3

so to solve c . . .

6c - 2d = 33
6c - 2 x -3 = 33
6c + 6 = 33
6c = 33 - 6
6c = 27
c = 27 / 6

soooooo..

d = -3
c = 4.5

Which all makes sense to me, however on the example that I did, (the next one) it contradicts it's self with the answer, this is my working:

2x - 3y = 11 - - - - 1
5x + 2y = 18 - - - - 2

10x - 15y = 55
10x + 4y = 36

-9y = 19

y = 19 / -9

y = -2.11

10x - 15y = 55
10x - 2 x -2.11 = 55
10x + 4.22 = 55
10x = 55 - 4.22
10x = 50.78
x = 50.78 / 10

sooooo:

x = 5.078
y = 2.11

Now the problem is that in the answer to the question just demonstrated, whilst subtracting the two equations from each other, they have subtracted in the reverse way that they did in the example at the start of this post, so whilst I did this:

10x - 15y = 55
10x + 4y = 36

-9y = 19

They did this (in the answer for this part of this question)

10x - 15y = 55
10x + 4y = 36

-19y = 19

y = -1


Which in turn changes the rest of the solution which they say is:

2x - 3 x -1 = 11
2x + 3 = 11
2x = 11 - 3
2x = 8
x = 8 / 2

which means that their solution to the equation is:

x = 4
y = 1

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I apologise for the long post but this has really stumped me and I would really like to start revising Surds and Circle Theorem but would rather perfect my Simultaneous Equation first haha

I thank everyone who helps me to solve this problem

 

[Deleted member 4993]

Hi there first post!

I have been doing some maths revision for my maths exam resit on monday and have been brushing up on Simultaneous Equations. I have been using the Maths Watch video's now I have found this program extremely useful until now however during the last topic, it seems to contradict its self.

the example was:

6c - 2d = 33 - - - 1
4c + 3d = 9 - - - 2

1 = x4
2 = x6

which then becomes:

24c - 8d - 132
24c + 18d = 54

therfore:

-26d = 78
d = 78 / -26
d = -3

so to solve c . . .

6c - 2d = 33
6c - 2 x -3 = 33
6c + 6 = 33
6c = 33 - 6
6c = 27
c = 27 / 6

soooooo..

d = -3
c = 4.5

Which all makes sense to me, however on the example that I did, (the next one) it contradicts it's self with the answer, this is my working:

2x - 3y = 11 - - - - 1
5x + 2y = 18 - - - - 2

10x - 15y = 55
10x + 4y = 36

-19y = 19

-15 - 4 = -19

y = 19 / -19 = -1

y = -2.11

10x - 15y = 55
10x - 2 x -2.11 = 55
10x + 4.22 = 55
10x = 55 - 4.22
10x = 50.78
x = 50.78 / 10

sooooo:

x = 5.078
y = 2.11

Now the problem is that in the answer to the question just demonstrated, whilst subtracting the two equations from each other, they have subtracted in the reverse way that they did in the example at the start of this post, so whilst I did this:

10x - 15y = 55
10x + 4y = 36

-9y = 19 (How did you get -9 when you subtracted 4 from negative 15)

They did this (in the answer for this part of this question)

10x - 15y = 55
10x + 4y = 36

-19y = 19

y = -1


Which in turn changes the rest of the solution which they say is:

2x - 3 x -1 = 11 <<< Don't do that --- very confusing
2x + 3 = 11
2x = 11 - 3
2x = 8
x = 8 / 2

which means that their solution to the equation is:

x = 4
y = -1


you can check validity of these answers by putting these back into the original equation:

2x - 3y = 11 - - - - 1
5x + 2y = 18 - - - - 2

for (1)

2(4) - 3(-1) = 8 - (-3) = 8 + 3 = 11 ....................checks

for (2)

5(4) + 2(-1) = 20 + (-2) = 20 - 2 = 18 ....................checks
-----------------------------------------------------------------------------------------------

I apologise for the long post but this has really stumped me and I would really like to start revising Surds and Circle Theorem but would rather perfect my Simultaneous Equation first haha

I thank everyone who helps me to solve this problem

 

[Denis]

10x - 15y = 55
10x + 4y = 36

-9y = 19

Noooooooo.....that's -19y = 19 :shock:

Why don't you do these the easy way:

2x - 3y = 11 [1]
5x + 2y = 18 [2]

10x - 15y = 55 [1]*5
-10x - 4y = -36 [2]*-2

-19y = 19

Get my drift?

 

[dosftw]

Argh! I feel so stupid now I see where I went wrong..

Thank you for your help