maths

calculate value of x for which f(x)= inverse f(x)

4/x+3=(4/x)-3

i got x to equal 4 as my answer but the answer in the book is 1 can anyone explain?

Reply 1

the bear

20

the RHS is not the inverse of the LHS :redface:

Reply 2

hhattiecc

9

Original post by the bear

the RHS is not the inverse of the LHS :redface:

I think OP means 4/(x+3) as f(x) and (4/x)-3 as its inverse. I don't know if there was some confusion with the way it was written, but I think it is the correct inverse.

OP could you post your working?

Reply 3

AndyOC

OP

2

Original post by hhattiecc

I think OP means 4/(x+3) as f(x) and (4/x)-3 as its inverse. I don't know if there was some confusion with the way it was written, but I think it is the correct inverse.

OP could you post your working?

yeah thats what i meant :smile:

its probably something really simple that im getting wrong..

added 3 to both sides to get 7/x+3=4/x
then multiplied both sides by x+3 to get 7=4x+12/x
multiplied both sides by x to get 7x=4x+12 and rearranged to get 3x=12

Reply 4

hhattiecc

9

Original post by AndyOC

yeah thats what i meant :smile:

its probably something really simple that im getting wrong..

added 3 to both sides to get 7/x+3=4/x
then multiplied both sides by x+3 to get 7=4x+12/x
multiplied both sides by x to get 7x=4x+12 and rearranged to get 3x=12

Please start using brackets around numerators and denominators haha, it's soooo hard to follow if you don't!

Originally when you went to add 3 to both sides, your sum should be [4/(x+3)] + 3, but you need the same denominator before you can add to a fraction (you can't just add it to the numerator)

Reply 5

hhattiecc

9

Also, are there some constraints on the value of x or does your book only give you one solution? I'm a bit confused as to why -4 isn't listed as a solution.

Reply 6

AndyOC

OP

2

Original post by hhattiecc

Also, are there some constraints on the value of x or does your book only give you one solution? I'm a bit confused as to why -4 isn't listed as a solution.

yeah there's only one solution the book is stupid really it's not uncommon for it to give a wrong answer as i've discovered plenty of times before

Thanks for the help anyway :smile:

it is simple

OP

Original post by BULL14

it is simple

yeah i've done it now just got confused with the basics haha

Reply 9

hhattiecc

9

Original post by AndyOC

yeah there's only one solution the book is stupid really it's not uncommon for it to give a wrong answer as i've discovered plenty of times before

Thanks for the help anyway :smile:

it's literally shocking how bad some of the a level books are in terms of giving correct answers.
No worries :smile:

maths

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