a = 2^1990 - 2^1989 - 2^1988
a = (2^2 - 2 - 1)* 2^1988
a = (4 - 2 - 1)* 2^1988
a = 1* 2^1988
a = 2^1988
2^1988 4^993
______ = _____
x 0,25
2^995
_____ = x = 2^997
x0,25
