#The solution in Python 3 is as follows:
from math import sqrt
t=int(input())
i=0
while i<t:
a, b, c, d = map(float, input().split())
s = (a+b+c+d)/2
print (round(sqrt(((s-a)*(s-b)*(s-c)*(s-d))),2)) #Brahmagupta's Formula
i+=1
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#Keep visiting for more solutions of SPOJ problems in Python 3.Thank you!!!
from math import sqrt
t=int(input())
i=0
while i<t:
a, b, c, d = map(float, input().split())
s = (a+b+c+d)/2
print (round(sqrt(((s-a)*(s-b)*(s-c)*(s-d))),2)) #Brahmagupta's Formula
i+=1
#Visit this link for details.
#Keep visiting for more solutions of SPOJ problems in Python 3.Thank you!!!
