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

[jake-the-peg]

Are those right? is there no = something in them?

[factor-fiction]

What is the question though Smowball?
These can't be 'solved' as there is no 'equals' in it.
I assume they are to be simplified.
So put the terms over a common denominator. I'll do one to show you what i think is needed.

1) (25y-x)/35xy

Is that what you mean?

[Smowball]

Sorry, forgot the = at the end of each! We need them solved! lol

[factor-fiction]

So if you have missed out the equals something bit, is it two simultaneous equations that we need to solve to find the values of x and y?
What year is he in?

[Smowball]

No, they just want the answer. Year 8

[Smowball]

Dont need value of x or y.

[jake-the-peg]

This can't be the whole formula they must be:

5/7x - 1/35y = something or other

or

1/7x + 3/x - 1/5y = something else

what are thes somethings on the other side of the equals signs?

[factor-fiction]

There is no answer as you have given us expressions not equations.

Is it a substitution problem? Have they been told to evaluate each expression for given values of x and y?

[katburd]

is this any help
http://answers.yahoo.com/question/index?qid=20110525235742AAGc26l

[jake-the-peg]

Oh - or it could be that there's an instruction like 'Express the following with whole numbers'?

we're not getting the whole question here

[factor-fiction]

If it's not substitution or solving it must be simplifying - adding fractions but with letters instead of numbers. I have done the first one

[Smowball]

Okay, I will write it as it is shown on the maths website, complete with the answer which we couldnt get.

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

[Smowball]

It is just solving - they want one answer for each.

[factor-fiction]

That's the answer I gave earlier.

[Smowball]

Sorry FF, I missed that - how did you get to that answer??

[factor-fiction]

It's not SOLVING because the expressions don't equal anything.
It's adding fractions in the normal way by creating a common denominator

[bert_h]

1st Q: put both terms over the commmon denominator 35xy.
5/7x = 25y/35xy
1/35y = x/35xy
therefore 5/7x - 1/35y = (25y - x) / 35xy

2nd Q: put all three terms over the common denominator 35xy
(that happens to be the same as in Q1, but you arrive at it differently)
1/7x = 5y/35xy
3/x = 105y/35xy
1/5y = 7x /35xy
therefore 1/7x + 3/x - 1/5y = (5y + 105y - 7x) / 35xy
= (110y - 7x) / 35xy

If these don't match the given answers, you should post a couple of the questions where you did get the right answer, so that we can try to work out what's expected.

[factor-fiction]

The denominator (bottom) needs to be something that 7x and 35y both 'go into'

The easiest one to use is the result of multiplying them together: 35xy

To put the first term over 35xy we need to multiply top and bottom by 5y.

To put the second term over 35xy we need to multiply top and bottom by x

[Smowball]

Oh my brain! lol.

[jake-the-peg]

factor's right they want them simplified

the secret here is that you can multiply the top or and the bottom of each term by any factor as long as you do the same to both

so 3/x is the same as (7x) 3 / (7x) x = 21x/7x²

and 1/7x is the same as x/7x²

and you can add the first two terms together over 7x²


Use this technique until you can get the same number on the bottom for each and then you can just combine them


any better?