I want to use Excel's RAND() function to calculate random points within
a circle. To do this, I want first to just calculate a random point on
the y-axis, less than the radius's value away from the center. That's
no problem.

But then I want to use those values generated in a second column, to
generate the correspondent random values in the x-axis. For instance,
the farther the y-value is from the center of the circle, the smaller I
can let the x-value be. The x-value becomes dependent on the y-value.

But every time I put in the calculation, the y-values change (based on
the volatility of the RAND() function), and the x-values are calculated
using the old y-values, and this creates several invalid points (i.e.
points outside of the circle).

Is there a way to get Excel to fix the random values in the first
column after it computes them, and subsequently use them to calculate
the values in the second column?