# Thread: I need some help

1. ## I need some help

Hi,
Somebody can tell me how I can solve the simultaneous equations for x and y

3x+2y=9
4x-6y=25

2. Originally Posted by maryluna0
Hi,
Somebody can tell me how I can solve the simultaneous equations for x and y

3x+2y=9
4x-6y=25
Hi maryluna0. Welcome to MHF.

What methods of solving this type of problem have you been taught? You might have learned substitution, addition and cancellation, or perhaps some matrix techniques.

Show me what you can do and I'll help you get it done.

3. hi mary,

you want to get two of the letters with the same number next to them ie you want the x's to be the same, so you find a number that both 4 and 3 go into- which is 12. you need to multiply four by three to get 12 and you need to multiply 3 by 4 to get 12. so multiply the whole of the first equation by 4 to get 12x + 8y = 36 then the whole of the second equation by 3 to get 12x - 18y = 75 then you subtract the first equation from the second (or the other way round it doesn't matter) to cancel out the x's to leave -26y = 39

4. Mark your explanation to poor Mary is quite difficult to follow written in that style.
Plus Mr Jameson (the Administrator) asked her to let us know what she knew first;
so that was a little impertinent of you to boomerang in as you did!

5. ok Administrator I did like this: 3x+2y=9
4x-6y=25
12x+8y=36
-12x+18y=-75
26=-39
/26=/26
I know thats is wrong,now could you tell me why please?

6. 12x + 8y = 36 is correct, but it should be 12x - 18y = 75 rather than "-12x + 18y = -75" you got the symbols mixed up.

so now you've got the two x values the same you can cancel them out by subtracting one of the equations from the other one eg

12x - 18y = 75 then take away
12x + 8y =36 from that

leaving you with -26y = 39

so y = - 1.5

7. Hi mary,
Here's my explanation,
3x+2y=9-------(i)
4x-6y=25------(ii)

Multiply (i) by 3,
therefore,

9x+6y=27---(i)(a)
4x-6y=25---(ii)

adding (i)(a) and (ii), we get,

13x=52

dividing by 13 on both sides, we hv,

x=4

Substitute for x in any one of the equations, say eqn (i),

then,

3(4) + 2y=9
or 2y=-3
or y=-3/2 or -1.5

this is ur soln set (4,-1.5)

8. Originally Posted by mark
12x + 8y = 36 is correct, but it should be 12x - 18y = 75 rather than "-12x + 18y = -75" you got the symbols mixed up.

so now you've got the two x values the same you can cancel them out by subtracting one of the equations from the other one eg

12x - 18y = 75 then take away
12x + 8y =36 from that

leaving you with -26y = 39

so y = - 1.5
Come on man, give her a break - "-12x + 18y = -75" is exactly as good as 12x - 18y = 75, you just arrive at it from a different angle.

All Mary needed to do was *add* her two equations.

What you get is 26y = -39.

I fear that what she wrote was 26 = -39.

All she did wrong was lose her y.

9. aadipotter!! Thank you very much for your clear explanation. very kind of you.

10. Originally Posted by maryluna0
aadipotter!! Thank you very much for your clear explanation. very kind of you.
No probs...

11. Mark thanks a lot..very kind