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2017-06-12 Intermediate


You trace a circle (the red one) using a pen with a thick circular end. Both the outside and inside borders of the trace are also circles (in blue):

If you trace any ellipse using the same kind of pen, will the outside and inside borders also be ellipses?

Imagine two stars AA and BB forming a binary star system, where they revolve around a common point in space under their mutual gravitational attraction alone. If AA is heavier than BB and they both revolve in circular orbits, then which star will have a smaller orbital radius?

The first few powers of 101 all begin with 1, as highlighted by red colors below: 1011=1011012=102011013=10303011014=104060401. \begin{aligned} 101^1 &=& {\color{#D61F06}{1}}01 \\ 101^2 &=& {\color{#D61F06}{1}}0201 \\ 101^3 &=& {\color{#D61F06}{1}}030301 \\ 101^4 &=& {\color{#D61F06}{1}}04060401 \\ &\vdots.& \end{aligned} Is that always the case for all powers of 101?

If a+a2+a3+a4+a5+a + a^2 + a^3 + a^4 + a^5 + \cdots is a positive number, then which of the following is larger,

a+a3+a5+a7+ora2+a4+a6+a8+?a+a^3+a^5+a^7+\cdots\quad \text{or}\quad a^2+a^4+a^6+a^8+\cdots\, ?

(1+1x)x+1=(1+11999)1999\large \left ( 1 + \dfrac{1}{x} \right )^{x+1} = \left ( 1 + \dfrac{1}{1999} \right )^{1999}

Find the sum of all real xx such that the above equation is true.


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