systems of equations and changing the coefficients

todd5821

New member
Joined
Jul 4, 2007
Messages
3
Here is the problem:

5a + 2b = -16
a + 3b = 15

I understand to multiply the second by 5, but when both equations have the same sign (add or subtract) I am having difficulty.
 

Denis

Senior Member
Joined
Feb 17, 2004
Messages
1,470
5a + 2b = -16
a + 3b = 15

Multiply by -5 instead:
5a + 2b = -16
-5a - 15b = -75

Now add: -13b = -91

That's a suggestion; I prefer adding to subtracting.
 

todd5821

New member
Joined
Jul 4, 2007
Messages
3
I have done that, but to solve for b I would divide by 13. 75 divided by 13 doesn't come out as a whole number. The answer book states that a=-6 and b=7. I haven't figured out how they got that!!
 

Mrspi

Senior Member
Joined
Dec 17, 2005
Messages
2,128
Please re-read Denis's earlier post.

When you add the two equations together, you don't get -75.....you get

-13b = -91

Now....divide both sides by -13.
 
Top