Answer: Look up your exact question
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The symmetric property of equality.
This property states that: if a = b, b = a.
We are given an equation:

This is an equation of a circle. General form of a circle equation is:

Where:
a = x coordinate of a center
b = y coordinate of a center
r = radius of a circle
Motion detector can detect person anywhere within a boundary. The greatest distance at which detector can detect is at the edge of a circle. That distance, between detector and edge of circle, is equal to radius.
From the equation we have:

The greatest distance at which person can be detected is 4ft.
Ten times the sum of two and five tenths and thirteen and 5 tenths.
Your answer would be 3.75.