4C + 57 4C  1 + A 1 + 35 2B 35 2.   1 + A 2B + 35  35  > 0 (...

2 .

Vậy min (abc) = 1995.

b) Đặt t = ^A = = = ^{B C D}

a b c d  A = ta, B = tb, C = tc, D = td.

t = A + B + C + D

a + b + c + d

Vì vậy aA + bB + cC + dD = a t + b t + c t + d t

²

²

²

²

= (a + b + c + d) t = (a + b + c + d) A + B + C + D

= (a + b + c +d)(A + B + C + D)