Thread: Solving Simultaneous Equations

I am doing some revision for my exams and I am really struggling with simultaneous equations. It's something I've never quite got my head around. I'm currently working through past papers and I've had a go at one of the questions, but there's no answers provided so I can't tell if I'm right or wrong. I reckon once I get my head around one I'll be fine with them.

This is the question:

When trying to solve the simultaneous equations 3x - y = 7 and 2x + 2y = 10 a student gets the answer x = 2, y = 2. For each of x and y, consider whether the value obtained is less than the correct value (too low), correct, or greater than the correct value (too high). Which of the following is true?

(A) The value x = 2 is too low and the value y = 2 is correct
(B) The value x = 2 is correct and the value y = 2 is too low
(C) The value x = 2 is correct and the value y = 2 is too high
(D) The value x = 2 is too high and the value y = 2 is too low
(E) None of the above is true

This is what I've done so far:

6x - 2y = 14
(6x - 2y) - 2x + 2y) = 4x = 14
x = 14 / 4 = 3.5

3 * 3.5 = 10.5
3 * 3.5 - y = 7
y = 3.5

...and now I'm stuck, I know I've gone wrong somewhere and I'm not quite sure where. I know there is the substitution method that I could try, but that's something I've never come across, although I might give it ago if it means I'll find it easier.

Thanks for any help in advance

Originally Posted by Gall1987

I am doing some revision for my exams and I am really struggling with simultaneous equations. It's something I've never quite got my head around. I'm currently working through past papers and I've had a go at one of the questions, but there's no answers provided so I can't tell if I'm right or wrong. I reckon once I get my head around one I'll be fine with them.

This is the question:

When trying to solve the simultaneous equations 3x - y = 7 and 2x + 2y = 10 a student gets the answer x = 2, y = 2. For each of x and y, consider whether the value obtained is less than the correct value (too low), correct, or greater than the correct value (too high). Which of the following is true?

(A) The value x = 2 is too low and the value y = 2 is correct
(B) The value x = 2 is correct and the value y = 2 is too low
(C) The value x = 2 is correct and the value y = 2 is too high
(D) The value x = 2 is too high and the value y = 2 is too low
(E) None of the above is true

This is what I've done so far:

6x - 2y = 14
(6x - 2y) - 2x + 2y) = 4x = 14
x = 14 / 4 = 3.5

3 * 3.5 = 10.5
3 * 3.5 - y = 7
y = 3.5

...and now I'm stuck, I know I've gone wrong somewhere and I'm not quite sure where. I know there is the substitution method that I could try, but that's something I've never come across, although I might give it ago if it means I'll find it easier.

Thanks for any help in advance

Your idea is sound but you have made a sign error




So first you multiply equation 1 by 2 to get



Now you add equation 3 to equation 2 to to get



Can you finish from here?

I'll give it a go...

I think you then put x = 3 in to equation 1?

So 3 * 3 - y = 7
Y = 2

So the answer to the original question would be A (The value x = 2 is too low and the value y = 2 is correct)?

Originally Posted by Gall1987

I'll give it a go...

I think you then put x = 3 in to equation 1?

So 3 * 3 - y = 7
Y = 2

So the answer to the original question would be A (The value x = 2 is too low and the value y = 2 is correct)?

Yes