Chapter 5: Problem 26
Let \(A B C\) be a triangle such that \(A C^{2}+A B^{2}=5 B C^{2}\). Prove that the medians from the vertices \(B\) and \(C\) are perpendicular.
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Chapter 5: Problem 26
Let \(A B C\) be a triangle such that \(A C^{2}+A B^{2}=5 B C^{2}\). Prove that the medians from the vertices \(B\) and \(C\) are perpendicular.
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Let \(z_{1}, z_{2}, \ldots, z_{n}\) be the coordinates of the vertices of a regular polygon with circumcenter at the origin of the complex plane. Prove that there are \(i, j, k \in\\{1,2, \ldots, n\\}\) such that \(z_{i}+z_{j}=z_{k}\) if and only if 6 divides \(n .\)
A function \(f: \mathbb{R}^{2} \rightarrow \mathbb{R}\) is called Olympic if it has the following property: given \(n \geq 3\) distinct points \(A_{1}, A_{2}, \ldots, A_{n} \in \mathbb{R}^{2}\), if \(f\left(A_{1}\right)=\) \(f\left(A_{2}\right)=\cdots=f\left(A_{n}\right)\), then the points \(A_{1}, A_{2}, \ldots, A_{n}\) are the vertices of a convex polygon. Let \(P \in \mathbb{C}[X]\) be a nonconstant polynomial. Prove that the function \(f: \mathbb{R}^{2} \rightarrow \mathbb{R}\) defined by \(f(x, y)=|P(x+i y)|\) is Olympic if and only if all the roots of \(P\) are equal.
Prove that if \(a, b, c\) are complex numbers such that $$ \left\\{\begin{array}{l} (a+b)(a+c)=b \\ (b+c)(b+a)=c \\ (c+a)(c+b)=a \end{array}\right. $$ then a, b, c are real numbers.
Let \(z_{1}, z_{2}, \ldots, z_{n}\) be distinct complex numbers such that $$ \left|z_{1}\right|=\left|z_{2}\right|=\cdots=\left|z_{n}\right|=1 $$ Consider the following statements: (a) \(z_{1}, z_{2}, \ldots, z_{n}\) are the coordinates of the vertices of a regular polygon. (b) \(z_{1}^{n}+z_{2}^{n}+\cdots+z_{n}^{n}=n(-1)^{n+1} z_{1} z_{2} \ldots z_{n}\) Decide with proof whether the implications \((a) \Rightarrow(b)\) and \((b) \Rightarrow(a)\) are true.
Let \(p \geq 3\) be a prime and let \(m, n\) be positive integers divisible by \(p\) such that \(n\) is odd. For each \(m\) -tuple \(\left(c_{1}, \ldots, c_{m}\right), c_{i} \in\\{1,2, \ldots, n\\}\), with the property that \(p \mid \sum_{i=1}^{m} c_{i}\), let us consider the product \(c_{1} \cdots c_{m} .\) Prove that the sum of all these products is divisible by \(\left(\frac{n}{p}\right)^{m}\).
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