maths

How do you do simultaneous equations?
if the signs are different you add , if the signs are same you subtract, however in some exam papers the answers are very different to what you work out?

Reply 1

Federerr

Original post by Ria25

How do you do simultaneous equations?
if the signs are different you add , if the signs are same you subtract, however in some exam papers the answers are very different to what you work out?

give us an example please and we'll go through it with you.

Reply 2

username2549677

Original post by Federerr

give us an example please and we'll go through it with you.

5x-3y= 11
4x-2y=18

Reply 3

username2549677

I mean't
5x+3y=11
4x-2y=18

Reply 4

Kvothe the Arcane

Original post by Ria25

I mean't
5x+3y=11
4x-2y=18

Have a look here.

You can either multiply the equations by a suitable factor to eliminate a variable or get one variable in terms of another from one equation and substitute it into the other.

Can you show us how you'd solve it? That way any error in your working can be highlighted.

Reply 5

username2549677

Original post by Kvothe the arcane

-3 x 2 = -6-3 x -2 = 66y + (-6y) = 0

5x + 2y = 11 x3 first eq.4x – 3y = 18 x2 second eq.15x+6y=33 8x-6y = 3623x = 69x = 3
5(3) + 2y = 1115 + 2y = 112y = -4 y = -2

Reply 6

Kvothe the Arcane

Original post by Ria25

-3 x 2 = -6-3 x -2 = 66y + (-6y) = 0

5x + 2y = 11 x3 first eq.4x – 3y = 18 x2 second eq.15x+6y=33 8x-6y = 3623x = 69x = 3
5(3) + 2y = 1115 + 2y = 112y = -4 y = -2

I'm afraid I have no idea what you're trying to write. Can you please reformat it? :confused:

See here regarding ambiguous notation.

(edited 7 years ago)

Reply 7

mphysical

Original post by Ria25

-3 x 2 = -6-3 x -2 = 66y + (-6y) = 0

5x + 2y = 11 x3 first eq.4x – 3y = 18 x2 second eq.15x+6y=33 8x-6y = 3623x = 69x = 3
5(3) + 2y = 1115 + 2y = 112y = -4 y = -2

Your working out is mind boggling.
I would multiply first equation by 2 and second equation by 3 then add them, leaving only the values of x.
Solve for x then substitute back in to one of the original equations.

Reply 8

username2549677

Original post by mphysical

yeah sorry , some ov it was just working out and the rest was de answers, but thank you really appreciate it , my maths exam is in 2 weeks.

Reply 9

username2549677

Original post by Kvothe the arcane

I'm afraid I have no idea what you're trying to write. Can you please reformat it? :confused:

See here regarding ambiguous notation.

5x + 2y = 11 x3 first eq.4x – 3y = 18 x2 second eq.15x+6y=33 8x-6y = 3623x = 69x = 3
5(3) + 2y = 1115 + 2y = 112y = -4 y = -2

sorry bout dat , is dis ok?

Reply 10

username2549677

sorry the question was
5x=2y+11
4x-3y=18

sorry i wrote the question wrong way around , the 2 on top and the three on the bottom , this is the right way.

5x+2y=11

5x+2y=11
4x-3y=18

First make an educated guess (so you know the answer you obtain after working it out is correct): x=3 and y=-2 works

We need to make the coefficient of x or y the same so we can use elimination. It is easier to make the coefficients of y the same since 2 and 3 are smaller and easier to multiply numbers with. We need to make the coefficient 6 since lcm(2,3)=6.

Label the equations:

5x+2y=11 - Equation A
4x-3y=18 - Equation B

Equation A *3 : 15x+6y=33
Equation B *2 : 8x-6y=36

Now lets add these two new equations to eliminate the y term:
23x=69
So x=3

Let's use equation A to find y by subbing in x=3: 5(3)+2y=11 so 15+2y=11, 2y=-4 and hence y=-2

So x=3 and y=-2 (which we guessed initially)