You are not logged in.

- Topics: Active | Unanswered

Pages: **1**

**righen****Guest**

Hi everyone,

Can someone help me solve this problem?

Solve this equation:

X^2 (x+3)=10x+24

Hence use a suitable substitution to solve

25x^2 (5x+3)=50x+24

The first part I was able to do! I obtain x=-2,-4, 3.

The second I am stuck.I dont know what to do?

Thank you in advance.

**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 7,913

hi righen

Welcome to the forum.

Your three answers for the first part look good to me.

So you have solved

or why not say you have solved

ie y = -2, -4 and 3.

I've made a new variable so I can use x as the variable in the second part.

If you put y = 5x in the above you get

which simplifies to

which is the second question.

So it must have solutions y = -2, y = -4 and y = 3

which becomes

5x = -2, 5x = -4 and 5x = 3

These will give three new values for x as answers.

Let's try out just one of these to make sure this is working. I'll take the third one.

If 5x = 3 then x = 3/5

Try in the equation

and

So LHS = RHS and we have a solution.

I'll leave you to work out the other two for yourself.

Bob

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

Offline

**righen****Guest**

Hi Bob,

Thank you for your quick reply and the nice welcome. You wont believe me I am actually doing a major in mathematics (its my first year) and decided to do some revision:D. To get stuck at such a low level problem is a shame.I just wanted to know what causes u to get to y=5x because I have to say I try my best to find a substitution but unfortunately I coudn't get it:(.what I try was to put both equation at 24 and equate the L.H.S of both equation to get a quadratic that give irrational roots:mad:.Anyway thank you again for the quick reply.;)

**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 7,913

hi righen

The clue is this:

Hence use a suitable substitution

and it helps that I've done loads of similar ones over the years.

I looked at the two equations and paired together the similar bits in my head like this:

x^2 (x+3) = 10x +24

25x^2 (5x+3) = 50x +24

I could see straight away that each x had become 5x and that was all I needed.

The person making up the question could have made it much nastier by multiplying out the brackets and moving the terms around a bit. So you will have to be on the lookout for that sort of thing too.

Thus instead of the second equation you could have been given:

But it's still the same problem.

Bob

Children are not defined by school ...........The Fonz

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

Offline

Pages: **1**