Hi,

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

3x+2y=9

4x-6y=25

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- September 3rd 2009, 08:54 AM #1

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- September 3rd 2009, 08:57 AM #2

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- September 3rd 2009, 10:18 AM #3

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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

- September 3rd 2009, 10:54 AM #4

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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!

- September 3rd 2009, 01:57 PM #5

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- September 3rd 2009, 02:08 PM #6

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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

- September 3rd 2009, 02:11 PM #7

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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)

- September 3rd 2009, 02:19 PM #8
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.

- September 3rd 2009, 02:37 PM #9

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- September 3rd 2009, 02:43 PM #10

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- September 3rd 2009, 03:07 PM #11

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