The letters $\mathrm{r}$ and $\theta$ represent polar coordinates. Write the equation using rectangular coordinates $(x, y)$. \[ r=5 \]

Step 1 ：The letters r and θ represent polar coordinates. We need to write the equation using rectangular coordinates (x, y).

Step 2 ：The given equation is r=5. In polar coordinates, r represents the distance from the origin to the point. Here, r is given as 5, which means the point lies on a circle of radius 5 centered at the origin.

Step 3 ：To convert this to rectangular coordinates, we can use the equation of a circle in rectangular coordinates, which is \(x^2 + y^2 = r^2\). Here, r is 5, so the equation becomes \(x^2 + y^2 = 25\).

Step 4 ：The equation in rectangular coordinates is \(\boxed{x^2 + y^2 = 25}\).