The equation for a circle is x2 + y2 = 25. What is the length of the circle's radius?

Question

jim_thompson5910 · Answer

Hint: x^2 + y^2 = r^2 is a circle with radius r

whpalmer4 · Answer

Jim's way is the straightforward way, but if you like tinkering a bit, try solving the equation for y = 0 and see what points you get.  Now use the distance formula or inspection to find the distance between the two points, and divide it in half. 
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anonymous · Answer

is it 12.5?

whpalmer4 · Answer

Mmm...no.  If $r^2 = 25$, $r = \sqrt{25}$, right?  What is the square root of 25?

zzr0ck3r · Answer

2a does not equal a^2

zzr0ck3r · Answer

ie square root of 25 is 5 not 12.5

anonymous · Answer

oh.. ok thanks (:

whpalmer4 · Answer

I may have confused you with the alternate solution.

anonymous · Answer

yea kinda but still, thank you (:

whpalmer4 · Answer

If we set y = 0, the equation reduces to $x^2=25$, which is true for $x = 5$ and $x = -5$.  The circle crosses the x-axis at (5,0) and (-5,0).  Those 2 points are 10 apart, and the line between them is a diameter, not a radius, so dividing it in half, we get a radius of 5 (just as we should).