Question 2347: I am a mom trying to help her high school daughter with her math; here is the question:
2c-d= -1
c+3d=17
Answer by longjonsilver(2297)   (Show Source):
You can put this solution on YOUR website! 2 equations...2 unknowns...is possible to solve.
The way i do things like this is to make 1 of the variables the same amount...my choice: i will make the c's the same by multiplying the second equation by 2 across the board.
This gives:
2c - d = -1
2c + 6d = 34
Now I will simply add or subtract the 2 equations. Which? Add or subtract? Look at the signs of the letter that has the same coefficient (the number in front)...for c, both are +ve, so remember the following:
SSSSame sign --> SSSSSubtract
DDDDifferent sign --> aDDDDDD. OK?
So, we subtract, to give -7d = -35. So d=(-35)/(-7) = 5
put this value into either of the original equations...again my choice, so i choose the easiest looking one ( the first one), to find c:
2c - 5 = -1
2c = 4
c = 2
Once you have the answers, ALWAYS put the answers into the other equation, to check that that one gives the answer you expect ie 17:
c+3d is 2+(3*5) is 2+15 is 17, whoo-hoo. It is correct but more importantly, I KNOW it is correct :-)
Look at all the other examples of simultaneous equations that i have done too on this website for more help if required.
jon.
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