Maths Question We Cant Solve.

[Smowball]

Part of son's homework was a section of 7 questions, all required the same method of solving. All done online. he did the same method for them all, 5 were marked correct, the last 2 were marked incorrect. We have all done the same method on these last 2 but all get a totally different answer to what the homework website says..

The 1st Q is : 5/7x - 1/35y

The 2nd Q is : 1/7x + 3/x - 1/5y

[factor-fiction]

Forget the letters for now:
If the first one was 5/7 - 1/35 you'd make 35 the common denominator so the expression would become (5x5)/35 -(1/35)= 24/35.

If he can't do that bit he needs to practise adding/subtracting numerical fractions before he tries these.

If he can add numerical fractions then just put in the extra steps needed to make the common denominator 35 x x[ix[i]y]= 35quote[xy]

Now do the same thing but take into account the extra terms

[Old_Geezer]

At the risk of making a fool of myself having not done this sort of thing for decades would this be the sort of thing they're looking for ?

5/7x - 1/35y = 25y - x/35xy.

Multiply all by x

5/7 - x/35y = 25xy - x/35y

25xy - x/35y + x/35y - 5/7 = 0

25xy - 5/7 = 0

25x = 5/7y

x = 0.028571429/y

y = 35 x ?

[Old_Geezer]

Ok yeah. (25/5) * 7 = 35. I could have jumped a step :-O

[Smowball]

Thankyou all so much for your answers - I really appreciate it. I think I get it(think being the key word!). I just need to now explain it to my son! : )

[albaqwerty]

good luck with that.
My head hurts from any kind of maths :-) x

[Smowball]

So does mine! lol

[factor-fiction]

Hi O_G,

You had a good go at this, but I think you went wrong early on.

I think you tried to start with my identity:
5/(7x) - 1/(35y) =(25y-x)/35xy

As the right hand side was just another way of writing the left hand side with a common denominator, we can't solve anything; any manipulation would simply lead to another identity such as 0=0 or 1=1

In terms of the manipulation I think yours went awry on the line after where you said "Multiply all by x "
You treated the term 25y as a standalone term whereas it should have a denominator of 35xy as it is actually part of the term (25y-x)/35xy

If you had included the denominator for the 25y term and then continued as before you'd have ended up with 5/7- x/(35y)= 25/35-x/(35y)
which simply tells us 5/7= 24/35

Hope you enjoy looking through it

[vascop]

Yes, I think that the original question should be written as follows to avoid confusion (Using BODMAS - yes that again):

5/(7x)-1/(35y)=(25y-x)/(35xy) or if you prefer to avoid brackets,

5/7/x-1/35/y=(25y-x)/35/x/y

[vascop]

Otherwise things like 5/7x would be interpreted as 5x/7 using BODMAS.

[Smowball]

And I thought this was going to be so easy...... lol