$ bc
bc 1.06
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a=1000000
t=16000
d=200000
.25*(a*t - sqrt(a)*sqrt(a*t^2 - 8*d))
250.00
If you evaluate each part of the formula you can see why this is probably not correct. \$a \cdot t^2 \$ is 256 trillion. \$8 \cdot d \$ is 1.6 million. Differences like this are bad news from a numeric stability perspective because since the minuend is so many orders of magnitude larger than the subtrahend, it only perturbs it by a little tiny bit. The last root will be extremely close to 16 million: 16 million - .05. That precision requires 9 decimal digits to represent, which is about equal to 30 binary digits: the mantissa of a single float only represents 23 bits. You need double floats to represent this quantity accurately.
What happens next makes it worse: the result is multiplied by a thousand, which makes it 16 billion - 50, and then it is subtracted from 16 billion. So from an orders of magnitude analysis, we start out with trillions and then cancel them out to zero.
This kind of formula is known as numerically unstable. There are worse examples, which don't even work with double floats, but you should re-organize the calculation so you aren't subtracting vast quantities that only slightly differ.