def check(a, b, c):
    return b**2 == a*c or 2*b == a+c

check(0.1, 0.2, 0.3)

check(0.600000000000000000000000000000003,0.700000000000000000000000000000006,0.800000000000000000000000000000009)

check(-1.0000000000000000000000000000000000000000000000000000001,0,1)

>>> 0.15*2==0.1+0.2
False

abs(b**2-a*c) < 1.0/2**3 or abs(2*b - a+c) < 1.0/2**3