Trying to change subject of equation using algebraic fractions

S

Simonsky

Junior Member
Joined
Jul 4, 2017
Messages
128
Hope I've chosen the right level to post this:

As an old returner to maths I'm practicing equations again and having a problem with this question using fractions:

Make 'R' the subject:

1/R = 1/R1 + 1/R2 .....................................[edited]

I had a go doing this which might be bizarre and incompetent: 1/R = R^-1 = R1^-1 + R2^-1

R^-1 x R^2 = R1^-1 X R^2 + R2^-1 +R^2 =

R = R^2(R1^-1 + R2^-1)

but that isn't the right answer!

So I then tried:

1/Rx R^2 = R^2/R = R so:

R = R^2/R1 + R^2/R2 then:

R = (R^2R2 + R^2R1)/R1R2

But that wasn't right! Help appreciated!
 
Last edited by a moderator:
Simonsky said:
Hope I've chosen the right level to post this:

As an old returner to maths I'm practising equations again and having a problem with this question using fractions:

Make 'R' the subject:

1/R = 1/R1 = 1/R2

I had a go doing this which might be bizarre and incompetent: 1/R = R^-1 = R1^-1 + R2^-1

R^-1 x R^2 = R1^-1 X R^2 + R2^-1 +R^2 =

R = R^2(R1^-1 + R2^-1)

but that isn't the right answer!

So I then tried:

1/Rx R^2 = R^2/R = R so:

R = R^2/R1 + R^2/R2 then:

R = (R^2R2 + R^2R1)/R1R2

But that wasn't right! Help appreciated!
Click to expand...
Welcome back Simonsky!

Please post the correct problem statement. You posted:

1/R = 1/R1 = 1/R2

I believe one of those '=' should be '+'

Please repost the problem - with the correction.
 
Sorry, should be, as you say:

1/R = 1/R1 + 1/R2

The rest is 'correct' (in its 'incorrectness', that is!)

Many thanks
 
Simonsky said:
Hope I've chosen the right level to post this:

As an old returner to maths I'm practising equations again and having a problem with this question using fractions:

Make 'R' the subject:

1/R = 1/R1 = 1/R2

I had a go doing this which might be bizarre and incompetent: 1/R = R^-1 = R1^-1 + R2^-1

R^-1 x R^2 = R1^-1 X R^2 + R2^-1 +R^2 =

R = R^2(R1^-1 + R2^-1)

but that isn't the right answer!

So I then tried:

1/Rx R^2 = R^2/R = R so:

R = R^2/R1 + R^2/R2 then:

R = (R^2R2 + R^2R1)/R1R2

But that wasn't right! Help appreciated!
Click to expand...
You are taking a very convoluted path and got lost. I would do it as follows:

1/R = 1/R1 + 1/R2 .......................... perform the addition of RHS

1/R = (R1 + R2) / (R1 * R2) ..............................flip both sides

R = (R1 * R2) / (R1 + R2) ............................. done
 
Thanks - doing things in a convoluted way is the story of my life!

Thanks-embarrassingly simple in the end!

Simon
 
JeffM said:
And all of us are lesser fleas upon the back of denis THE GREAT

Great fleas have little fleas upon their backs to bite 'em
And little fleas have lesser fleas and so ad infinitum
Click to expand...
And the great fleas themselves, in turn, have greater fleas to go on; While these again have greater still, and greater still, and so on.

or

The Vermin only teaze and pinch
Their Foes superior by an Inch.
So, Nat'ralists observe, a Flea
Hath smaller Fleas that on him prey,
And these have smaller yet to bite 'em,
And so proceed ad infinitum:
Thus ev'ry Poet, in his Kind
Is bit by him that comes behind
 
You must log in or register to reply here.
Top