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checking maths answers

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mollykins | 07:53 Fri 04th Jun 2010 | Science
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Factor, and anyone else good at maths, i've done some stuff but can you check them please, I would get my mum or dad or friend to but; i lost mum and dad with most my work in year 9 and my friends are only doing foundation.

1) these are eqations of lines; (P) y=3x+5 (Q) y=4-3x (R) y+3x=1

a) which is parallel to y=3x ? P and Q

b) which goes through the point (2,7) ? S

c) write down the gradient of y+2x=7 ? +7 (is it necessary to say + ?)


And this at the start of a paper but i always find explaining things hard.

2) mike says that 18% or £840 is equal to 36% of £420

explain why he is correct, do not calculate 18% of £840.

18% is half of 36%. and 840 is double 420.

2 (n/2) = n ?????????? i have no idea if that is right.

3)make t the subject of the formula w=2t+v

-v . . . . . w-v=2t
1/2 . . . . (w-v)/2=t

when doing stuff liek that above, i normally draw a vertical line and and jot down my stages, similar to what i've done there, is that a good idea? When checking back over, its easier to spot mistakes i think and its clear to the examiner what i was doing.
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hi molly-wot about ye
1.) Only P is parallel. Q has a negative gradient (-3) and is perpendicular to y=3x
1b) What is the equation of line S. Certainly P,Q and R don't pass through those points
1c) No. You can rewrite this as y= -2x +7 so gradient is -2. The 7 is not the gradient- it's where the line intercepts the y axis
Question Author
oops, s is y-3x=1
2. Yes.
Back to Q1 then,
If S is y-3x=1 then y= 3x+1.
So for a) S is also parallel to y=3x
And (2,7) does lie on S
3) w=2t+v
so w-v=2t
so 2t= w-v
so t= (w-v)/2
Exactly as you've done it
Question Author
:D the last one is the most i'm unsure about.
Question Author
I mean the middle one.
Yes, always explain what you are doing. Write each step separately (many pupils just write an endless stream of nonsense calculations such as 1+1=2 x3=6 +100 =106
I suggest you 'tell a story' to the examiner about what you are doing (eg "subtracting v from both sides"), but in your case be careful as I know you can go on a bit sometimes, lol).
Question Author
And if i can i lay each stage on a seperate line.
Mike says that 18% or £840 is equal to 36% of £420. Explain why he is correct; do not calculate 18% of £840.

Well you could say 18% of £840 = 840 x 18/100
840= 2 x 420
18 x 2 =36

so 18% of £840 = 840 x 18/100 can be rewritten as 2 x 420 x (36/2)/100

which in turn can be rewritten thus:
2 x 420 x (36/2)/100 = 420 x 36/100.

Now 36% of £420 = 420x 36/100

The two are identical.

But that's a bit long winded isn't it.

If it's worth only one mark I'd go for your explanation- they simply want you to show you know that y% of 2n is the same as 2y% of n
Yes, always put each step on a separate line.
Is that it?
My bill is in the post.
could you go through again starting from 'negative gradient'?
Question Author
thanks factor [sends cyber hugs and kisses] my maths exams which are 55% of my gcse grade are on monday and friday
Question Author
positive gradient goes from bottom left to top right and negative goes from top left to bottom right.
Correct mollykins. I'm sure bibblebub was only teasing and I expect he is pretty good at this stuff- my guess is he has done maths beyond GCSE
AQA module 5 mollykins?
Question Author
3 and 5 so we have 2, 90 minute exams monday is non calc and friday is calcs allowed.

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