changing the subject (maths)

[christy053]

can anyone help me figure out these changing the suject math problems ? Make x the subject of the following t=x+8 3x=z x over 5 =t c=x-7b ta xxx

[Buenchico]

I'm trying to work out which year you (or possibly your child) is in at school. Anyway, this is definitely the only time I'll do your homework for you!

If you want to pass GCSE maths (which I assume is where this will eventually lead), the important thing you need to know is that when x is 'tied up' with something else, and you want to leave x on its own, then you simply need to 'undo' the bit you don't want (and do the same thing to the other side of the equation).

If t = x + 8, the x would be on its own if it wasn't for the annoying fact that someone has added 8 to it. So what do you do? Simply take 8 away. Doing this to both sides of the equation, gives t - 8 = x.
That's your answer, except that, for the sake of neatness it's best to write it the other way round: x = t - 8.

If 3x = z, x would be on its own if some annotying person hadn't come along and multiplied it by 3. So how do we undo this? Well, the opposite of multiplying by 3 is to divide by 3, so thats what we do to both sides of the equation:
x = z/3. (That's 'z over 3')

If x over 5 = t, x would be on its own if it wasn't for the 'over 5' bit (which means that its beeen divided by 5). So we need to undo that bit. How? Well the opposite of dividing by 5 is to multiply by 5, so that's what we do to both sides of the equation:
x = 5t

Lastly, if c = x - 7b, the x would be on its own if it hadn't had 7b taken away from it. So, to undo that, we need to add 7b to both sides of the equation:
c + 7b =x
That's your answer but, once again, for neatness, it's best to write it the other way round:
x = c + 7b

IMPORTANT: Copying down those answers will get your homework right but if you don't understand the reasoning, you need to ask your teacher. This type of algebra forms the foundation for the vast majority of the rest of the GCSE maths course. If you don't really

[Buenchico]

. . . understand how to do this, over two-thirds of the GCSE maths course will make no sense to you.

'Cheating' now (by getting the answers from someone else, without bothering to understand the reasoning) could lead to failure later on.

If you don't understand, ask your teacher. If you still don't understand, ask again. If you still don't understand, ask another teacher or ask someone else who can explain.

This part of mathematics is really important. If you don't know how to do this, you'll fail, fail and fail again all the way to your GCSE exam. Don't cheat! Ask!

Chris
(Former maths teacher)

[NACL]

good job in explaining! (current math teacher) :D