# Tagged: positive semi-definite matrix

## Problem 559

For each of the following matrix $A$, prove that $\mathbf{x}^{\trans}A\mathbf{x} \geq 0$ for all vectors $\mathbf{x}$ in $\R^2$. Also, determine those vectors $\mathbf{x}\in \R^2$ such that $\mathbf{x}^{\trans}A\mathbf{x}=0$.

(a) $A=\begin{bmatrix} 4 & 2\\ 2& 1 \end{bmatrix}$.

(b) $A=\begin{bmatrix} 2 & 1\\ 1& 3 \end{bmatrix}$.

## Problem 514

Prove that a positive definite matrix has a unique positive definite square root.

## Problem 513

Let $A$ be a square matrix. A matrix $B$ satisfying $B^2=A$ is call a square root of $A$.

Find all the square roots of the matrix
$A=\begin{bmatrix} 2 & 2\\ 2& 2 \end{bmatrix}.$