#### Posers and Puzzles

1.  astevenson
i wish i was BOB!!!!
15 Jun '03 19:22
can someone help me here, this was in my GCSE maths paper and i didnt have a clue how to solve it..

prove that 20x + x squared = 400 (prove means u dont answer it im guessing)
2. 17 Jun '03 00:21 / 1 edit
I cannot see how you're supposed to prove that:
20x + x^2 = 400

However, it is possible to solve it, and find x (I suppose that's the purpose of this task):

20x + x^2 = 400
x^2 + 20x + 100 = 500
(x + 10)^2 = 500
x + 10 = [+/-]sqrt(500)
x = [+/-] 10sqrt(5) - 10

If you'd like, you can say:
x = 10([+/-]sqrt(5) - 1)

[+/-] -- plusminus-thingy (\pm in (La)TeX)
3. 10 Jul '03 15:24
maybe it was

prove that 20x squared divided by x^2 = 400
4.  genius
Wayward Soul
11 Jul '03 12:42
Originally posted by astevenson
can someone help me here, this was in my GCSE maths paper and i didnt have a clue how to solve it..

prove that 20x + x squared = 400 (prove means u dont answer it im guessing)
if you solve it, then you are proving it, are you not? but would that not be possible for any equation of that form? i.e. 463x+2x^2=547, albeit that x will not always be in interget...?
5. 12 Jul '03 21:38
Maybe it is possible if the x was something on the lines of .9975124???? (I run out digits on my calc)
6. 12 Jul '03 21:45 / 1 edit
Did in excel instead x= 0.997512422417805
7. 12 Jul '03 23:21
Originally posted by astevenson
can someone help me here, this was in my GCSE maths paper and i didnt have a clue how to solve it..

prove that 20x + x squared = 400 (prove means u dont answer it im guessing)
For the wording of that question to make sense, they must have given you the value of X or made you work it out in an earlier part of the question.