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Simultaneous Question

 
 
Reply Tue 22 Jan, 2013 09:09 am
Hi people , i need help on the following simultaneous equations Smile :

1) 3p = 2q
4p +q + 11 = 0

2) 2.5x + 0.75 - 3y = 0
1.6x = 1.08 - 1.2y

3) 7p = 11 + 2q = 0
-1 = 3q- 5p
 
Kolyo
 
  3  
Reply Tue 22 Jan, 2013 04:04 pm
@Confused student,
Quote:
1) 3p = 2q
4p +q + 11 = 0


Use substitution. First try to figure out how many p's one q is worth.

If 2 q's are worth 3 p's. How much is one worth? Answer: q = 1.5p.
(That's called "solving for q.")

So where you see q in the second equation, you can substitute 1.5p:
4p + 1.5p + 11 = 0.

Combining like terms gives you 5.5p + 11 = 0.
Solving for p gives you p = -2.

Now put the value of p back into the first equation:
3(-2) = 2q.

Solving for q gives us: q = -3.

----------------------------
Summary of overall strategy:
--Figure out what q is worth in terms of p, using the first equation.
--Rewrite the second equation, putting some multiple of p where q is.
--Solve the second equation for p.
--Figure out q.
Confused student
 
  1  
Reply Tue 22 Jan, 2013 04:43 pm
@Kolyo,
Thank you so much for helping me Smile
0 Replies
 
raprap
 
  2  
Reply Fri 25 Jan, 2013 02:08 am
@Confused student,
Another common method is called elimination

e.g.
3p = 2q
4p +q + 11 = 0

becomes
3p - 2q=0
4p +q + 11 = 0

four times the first equatio and triple the second becomes
12p-8q=0
12p+3q + 33 = 0

Subtract the first from the second

11q+33=0

rearrange

11q=-33

and solve

q=-3

so p=-2

now check the answer

3(-2) = 2(-3) check
4(-2) +(-3) + 11 =-8-3+11=-(8+3)+11=-11+11=0 check

BTW elimination is better done by using matrix notation

3p = 2q
4p +q + 11 = 0

becomes
[3 -2] [p] [ 0]
[4 1] [q] [-11]

Rap



Confused student
 
  1  
Reply Tue 29 Jan, 2013 10:58 am
@raprap,
So for simultaneous equations you can use the elimination method as well, thanks for the reply Smile
raprap
 
  2  
Reply Tue 29 Jan, 2013 11:24 am
@Confused student,
Yes

There also a graphical method

the two equations
3p = 2q
4p +q + 11 = 0

are also lines in thd p:q plane
q=3/2p and q=-4p-11

if you were to plot these two lines in this plane thery would intersect ar a point (p0,q0) . This point would be the solution to the simultaneous equations.

Rap
Confused student
 
  1  
Reply Tue 29 Jan, 2013 12:09 pm
@raprap,
Cool thanks for showing me both methods Smile
0 Replies
 
 

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