A=5^1992+1/2*5^1990+3/2*5^1991+5
=5^1992+5^1990*(1/2+3/2*5)+5
=5^1992+5^1990*8+5
=5^1990*(5^2+8)+5
=5^1990*33+3+2
所以A被3除的余数是2