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jxharmon

Member

Registered: 2007-01-08

Posts: 104

solve system by addition

2x+3y=1 and 5x+3y=16 state whether incon sistent or dependent; if I multiply the first equation by -1, Iget -2x-3y=-1 which means x=5 but then when I plug that into 5(5) +3y=16; 25+3y=16; 3y=-25+16; 3y=9;y=3   2(5) +3(3) =1 10 +9=1?   5(5) +3(3)=16 25+9=16 what am I doing wrong?

Thomas11

Member

Registered: 2007-02-01

Posts: 15

Re: solve system by addition

Here's the solution:

2x+3y=1
5x+3y=16 | 3x=15, x=5

5*5+3y=16
25+3y=16
3y=-9
y=-3

x=5
y=-3

Last edited by Thomas11 (2007-02-02 03:11:04)

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: solve system by addition

That's nearly right, you just dropped a minus sign when working out y.
3y = -25 + 16 = -9 --> y = -3.

And then checking the equations gives you that 2*5 + 3*(-3) = 1 and that 5*5 + 3*(-3) = 16, both of which are right.

I've never learnt about simultaneous equations being classed as inconsistent or dependent, so I can't help you there, I'm afraid.

---

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It wanted to be normal.

jxharmon

Member

Registered: 2007-01-08

Posts: 104

Re: solve system by addition

thanks I guess I just overlook the minus great help from both