# Problems of the Week

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# 2018-05-14 Intermediate

Wild tic-tac-toe is like traditional tic-tac-toe except that on each turn, a player may place an X or an O. The first person to get a row, column, or diagonal of three Xs or Os wins the game.

If Wild tic-tac-toe is played optimally, who should win?

This particular game ends on victory for player 2. The moves (and which player made them) are marked.

\begin{align} 1+1 \times 2 \times 3 \times 4 &= 5^2 \\ 1+2 \times 3 \times 4 \times 5 &= 11^2 \\ 1 + 3 \times 4 \times 5 \times 6 &= 19^2 \end{align}

Is 1 plus the product of four consecutive integers always a perfect square?

This bowl is in the shape of the paraboloid $$z=x^2+y^2$$. A ball falls to the bottom such that it touches the lowest point $$(0,0,0)$$ of the bowl.

What is the maximum possible radius of the ball?

Two $$\SI{1}{\kilo\gram}$$ beads are at the ends of a compressed spring resting on a smooth parabolic wire $$\big(y = x^2\big).$$ The spring has a natural length of $$\SI{1}{\meter}$$ and a force constant of $$\SI[per-mode=symbol]{5}{\newton\per\meter}.$$

If the system is in static equilibrium, how far away from the $$y$$-axis is each bead (in meters)?

Note: The spring is horizontal and gravity is $$\SI[per-mode=symbol]{10}{\meter\per\second\squared}$$ in the $$-y$$ direction.

$012345678910{\color{blue}111}21314\ldots$

The above is the beginning of all non-negative integers concatenated into a single infinite string.

$$\color{blue}111$$ is the first sub-string with 3 identical digits in a row, with different digits preceding and following the run of the identical digits. The first $$\color{blue}1$$ of $$\color{blue}111$$ comes at position 13 (from left) in the infinite string.

Where is the first such sub-string with 4 identical digits?

Give the position of the first digit of the four.

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