# Australian Maths Competition (1 Viewer)

#### Mongoose528

##### Member
NEW Q: . Small squares of side x cm have been removed from the corners, sides and centre of a square of side y cm to form the gasket shown.
If x and y are prime numbers and the sum of the inside and outside perimeters of the gasket, in centimetres, is equal to the area of the gasket, in square centimetres, what is the smallest possible value of the area of the gasket?

#### Mongoose528

##### Member
http://imgur.com/a/uDxq1

you found the diameter of a circle with area 16pi, not the diameter of the tube
Thanks, I should definitely draw a picture next time so I don't get it wrong. Can't make mistakes like this in the competition on Thursday.

#### dan964

##### MOD
Moderator
NEW Q: . Small squares of side x cm have been removed from the corners, sides and centre of a square of side y cm to form the gasket shown.
If x and y are prime numbers and the sum of the inside and outside perimeters of the gasket, in centimetres, is equal to the area of the gasket, in square centimetres, what is the smallest possible value of the area of the gasket?
needs a diagram

Here:

#### 1729

##### Active Member
NEW Q: . Small squares of side x cm have been removed from the corners, sides and centre of a square of side y cm to form the gasket shown.
If x and y are prime numbers and the sum of the inside and outside perimeters of the gasket, in centimetres, is equal to the area of the gasket, in square centimetres, what is the smallest possible value of the area of the gasket?
\bg_white \noindent The area of the gasket is A = y^2 - 9x^2 \\ The perimeter of the gasket is P = 4(y-3x) + 24x = 4y + 12x. So, \\ \begin{align*} y^2 - 9x^2 &= 4y + 12x \\ (y-3x)(y+3x) &= 4(y+3x) \\ y - 3x &= 4 \text{ since } y+3x \neq 0 \end{align*} \\ If x = 2, y = 10 which is not prime but if x = 3, y = 13 which is prime so the least area is trivially A = 13^2 - 9(3)^2 = 88 cm^2

#### Mongoose528

##### Member
\bg_white \noindent The area of the gasket is A = y^2 - 9x^2 \\ The perimeter of the gasket is P = 4(y-3x) + 24x = 4y + 12x. So, \\ \begin{align*} y^2 - 9x^2 &= 4y + 12x \\ (y-3x)(y+3x) &= 4(y+3x) \\ y - 3x &= 4 \text{ since } y+3x \neq 0 \end{align*} \\ If x = 2, y = 10 which is not prime but if x = 3, y = 13 which is prime so the least area is trivially A = 13^2 - 9(3)^2 = 88 cm^2
Nicely done, want to ask a new question?

#### 1729

##### Active Member
Nicely done, want to ask a new question?
From where?

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#### Mongoose528

##### Member
From where?

Here's a link to some past papers:

Good luck!

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#### si2136

##### Well-Known Member
@Mongose @1729

Are you both doing AMC in the senior division?

#### dan964

##### MOD
Moderator
Just a friendly reminder, that AMC papers are copyrighted. While we permit posting of individual questions at a time, posting sites that host AMC (Australian Maths Competition), AIMO papers is not allowed.

#### Mongoose528

##### Member
@Mongose @1729

Are you both doing AMC in the senior division?
Nah, I'm doing the intermediate. I don't think they vary too much in difficulty though as I've noticed that question 30 in the intermediate is around question 28/29 in senior. I've also noticed that their are quite a few common questions each year.

#### pikachu975

##### I love trials
Moderator
The bottom of the circle travels half a circumference, i.e. pi*r

Then you add on 2 radii (bottom of circle to point X for both sides) so 2r

distance = pi*r + 2r
= r(pi+2) (B) I guess

#### Mongoose528

##### Member
20.

Q: If x and y are positive integers which satisfy x^2 - 8x -1001y^2 = 0, what is the smallest possible value of x + y?

I wasn't sure on how to do that question, thanks.

#### 1729

##### Active Member

I wasn't sure on how to do that question, thanks.
Exactly six of these 'unusually shaped portions' fit into the regular octahedron by symmetry (one at each vertex) so the volume of one portion is one-sixth the volume of the octahedron, ie. 1/6 * 120 = 20

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#### Mongoose528

##### Member
Here are 3 more to keep you going through the day:

#### seanieg89

##### Well-Known Member
Let f(n) be the quantity for a given n. It is pretty easy to show that f(mn)=f(m)f(n) for coprime m and n (a and b each uniquely factorise into the product of something that divides m and something that divides n), and also that f(p^n)=(n+1)(n+2)/2 by manual counting.

Hence f(6^6)=f(2^6)f(3^6)=28^2=784.

Note that this method makes it easy to compute f(n) for an arbitrary n that we know the prime factorisation of.

#### Jonothancroller

##### Member
Hey, I've got some questions that I haven't been able to do. Would anyone care to do these questions as practice and offer an explanation? Thanks.

http://imgur.com/a/YfigO

#### Jonothancroller

##### Member
Notice that the unshaded area must also be divided in half. By drawing a diagram with the centres of the circles and the perpendiculars from the centres to the dividing line, it can be shown that the line must pass through the midpoint between the centres of the spheres. Letting P be the origin and then have a normal x-y plane, M = (5/2, 1) and also passes through C = (6,2). Then, find the line MN and substitute x=0 to obtian y = 2/7, which is A.