To find distance between points $ A(x_1,y_1)$ and $ B(x_2,y_2)$, we use formula:

$$ \color{blue}{ d(A,B) = \sqrt{(x_2 - x_1)^2+(y_2 - y_1)^2} } $$

In this example we have:

$$ \begin{aligned} & A \left(1200,~1600\right) \implies x_1 = 1200 ~~\text{and}~~ y_1 = 1600 \\[1 em] & B \left(0,~0\right) \implies x_2 = 0 ~~\text{and}~~ y_2 = 0 \end{aligned} $$

Substituting $ x_1 $, $ x_2 $, $ y_1 $ and $ y_2 $ into the formula above yields:

$$ \begin{aligned} d(A,B) & = \sqrt{\left( 0 - 1200 \right)^2 + \left( 0 - 1600 \right)^2} \\[1 em] d(A,B) & = \sqrt{ (-1200)^2 + (-1600)^2 } \\[1 em] d(A,B) & = \sqrt{ 1440000 + 2560000 } \\[1 em] d(A,B) & = \sqrt{ 4000000 } \\[1 em] d(A,B) & = 2000 \end{aligned} $$

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