Solve

1. Find all sets of four real numbers x1,x2,x3,x4 such that the sum of any one and the product of the other three is equal to 2.

2. Prove that cos(π/7) - cos(2π/7) + cos(3π/7) = 1/2 .

3. Solve the equation

cos^n{x} - sin^n{x} = 1

where n is a given positive integer.

寒い！寂しい？

気づけば、寒い2月もあと1週間ですね！