how to use math.sqrt ()?

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Python

hi guys, i'm beginner of python, i trying use math.sqrt() to count distance.

print math.sqrt(float(data[0][0]) - float(k_cluster[0][0]**2) + float(data[0][1]) - float(k_cluster[0][1]**2)))

for example:

data[0] = 12.1
data[0][0] = 15.125

data[0][1] = 13.214

k_cluster[0][0] = 5.34
k_cluster[0][1] = 6.78

when i use code above it tell me math domain error, but if without math.sqrt() it work normally
do you know what the problem??