... 2 a - < /b> ab + b m a - < /b> 2ab + b 0(ỳng) a + ab +b 2 a - < /b> ab + b (a + b ) => (a + b) a + ab +b 3 1 < /b> b3 + c3 c + a Tngt: ) (b + c ) (c + a 2 3 b + bc + c c + ca +a => P ( a + b + c ) abc =2(BTCụsi) ... 12< /b> < /b> 3 Cõu5 Cúx,y,z>0,t:a=x ,b= y ,c=z (a ,b, c>0abc =1)< /b> (1im) a + b3 b3 + c3 c + a + + Tacú: P= a + ab + b b + bc + c c + ca +a 0.25 0.25 0.25 0.25 a - < /b> ab + b a + b3 = (a + b ) a + ab +b a + ab + b ... + ( y - < /b> 3) =25 0.25 0.25 Cõu 7b Tacú n ( W ) =12< /b> !< /b> (1im) Gi Albin cNhúm hcsinhxpthnhhngngangsaocho4hskhi 12< /b> ngcnh nhau, 3hcsinhkhi 11< /b> ngcnh Tacoi4hcsinhkhi 12< /b> nh1phn ta,3hc sinhkhi 11< /b> nhphn tb. 0.25...