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The equations \[ax^2 + bx + c = 0 \tag{1}\] and \[- ax^2 + bx + c \tag{2}\] are given. Prove that if \(x_1\) and \(x_2\) are, respectively, any roots of the equations (1) and (2), then there is a root \(x_3\) of the equation \(\frac 12 ax^2 + bx + c\) such that either \(x_1 \leq x_3 \leq x_2\) or \(x_1 \geq x_3 \geq x_2\).

Prove that if \(x_0^4 + a_1x_0^3 + a_2x_0^2 + a_3x_0 + a_4\) and \(4x_0^3 + 3a_1x_0^2 + 2a_2x_0 + a_3 = 0\) then \(x^4 + a_1x^3 + a_2x^2 + a_3x + a_4\) is divisible by \((x - x_0)^2\).

At what value of \(k\) is the quantity \(A_k = (19^k + 66^k)/k!\) at its maximum?

The number \(n\) has the property that when it is divided by \(q^2\) the remainder is smaller than \(q^2 / 2\), whatever the value of \(q\). List all numbers that have this property.

We consider a function \(y = f (x)\) defined on the whole set of real numbers and satisfying \(f (x + k) \times (1 - f (x)) = 1 + f (x)\) for some number \(k \ne 0\). Prove that \(f (x)\) is a periodic function.

Find the minimum for all \(\alpha\), \(\beta\) of the maximum of the function \(y (x) = | \cos x + \alpha \cos 2x + \beta \cos 3x |\).