In a 10-storey house, 1 person lives on the first floor, 2 on the second floor, 3 on the third, 4 on the fourth, ..., 10 on the tenth. On which floor does the elevator stop most often?
Two expressions are written on the board:
\[1 + 22 + 333 + 4444 + 55555 + 666666 +7777777 + 88888888 + 999999999\] \[9 + 98 + 987 + 9876 + 98765 + 987654 + 9876543 + 98765432 + 987654321\]
Determine which one is greater or whether the numbers are equal.
Let \(r\) be a rational number and \(x\) be an irrational number(i.e. not a rational one). Prove that the number \(r+x\) is irrational.
If \(r\) and \(s\) are both irrational, then must \(r+s\) be irrational as well?
Definition: We call a number \(x\) rational if there exist two integers \(p\) and \(q\) such that \(x=\frac{p}{q}\). We assume that \(p\) and \(q\) are coprime.
Prove that \(\sqrt{2}\) is not rational.
Let \(n\) be an integer. Prove that if \(n^2\) is divisible by \(2\), then \(n\) is divisible by \(2\).
Let \(n\) be an integer. Prove that if \(n^3\) is divisible by \(3\), then \(n\) is divisible by \(3\).
The numbers \(x\) and \(y\) satisfy \(x+3 = y+5\). Prove that \(x>y\).
The numbers \(x\) and \(y\) satisfy \(x+7 \geq y+8\). Prove that \(x>y\).
Prove that there are infinitely many natural numbers \(\{1,2,3,4,...\}\).
Is it possible to colour the cells of a \(3\times 3\) board into red and yellow such that there are the same number of red cells and yellow cells?