Problems

Let \(a\) be a positive integer, and let \(p\) be a prime number. Prove that \(a^p - a\) is a multiple of \(p\).

We ‘typically’ use the formula \(\frac{1}{2}bh\) for the area of a triangle, where \(b\) is the length of the base, and \(h\) is the perpendicular height. Here’s another one, called Heron’s formula.

Call the sides of the triangle \(a\), \(b\) and \(c\). The perimeter is \(a+b+c\). We call half of this the textitsemiperimeter, \(s=\frac{a+b+c}{2}\). Then the area of this triangle is \[\sqrt{s(s-a)(s-b)(s-c)}.\] Prove this formula is correct.

Let \(\phi(n)\) be the Euler’s function, namely the amount of numbers from \(1\) to \(n\), coprime with \(n\). For two natural numbers \(m,n\) such that \(\mathbb{GCD}(m,n)=1\) prove that \(\phi(mn) = \phi(m)\phi(n)\).

All of the rectangles in the figure below, which is drawn to scale, are similar to the big rectangle (that is, their sides are in the same ratio). Each number represents the area of the rectangle. What is the length \(AB\)?

For any positive integer \(k\), the factorial \(k!\) is defined as a product of all integersbetween 1 and \(k\) inclusive: \(k! = k \times (k − 1) \times ... \times 1\). What’s the remainder when \(2025!+2024!+2023!+...+3!+2!+1!\) is divided by \(8\)?

Find all functions \(f\) from the real numbers to the real numbers such that \(xy=f(x)f(y)-f(x+y)\) for all real numbers x and y.

There are two imposters and seven crewmates on the rocket ‘Plus’. How many ways are there for the nine people to split into three groups of three, such that each group has at least two crewmates? The two imposters and seven crewmates are all distinguishable from each other, but we’re not concerned with the order of the three groups.

For example: \(\{I1,C1,C2\}\), \(\{I2,C3,C4\}\) and \(\{C5,C6,C7\}\) is the same as
\(\{C3,C4,I2\}\), \(\{C5,C6,C7\}\) and \(\{I1,C2,C1\}\) but different from
\(\{I2,C1,C2\}\), \(\{I1,C3,C4\}\) and \(\{C5,C6,C7\}\).