1.cho $a,b,c$ dương thỏa mản: $21ab+2bc+8ca\leq 12$
Tìm $\min P= \frac{1}{a}+\frac{2}{b}+\frac{2}{c}$
2.cho $a,b,c$ dương thỏa mản $\frac{1}{a}+\frac{4}{b}+\frac{9}{c}=1$
     cm: $9a+b+4c \geq 121$
3.cho $a,b,c$ dương thỏa mản $2abc=3a^2+4b^2+5c^2$
Tìm $\min P=3a+2b+c$
4.cho $a,b,c$ dương tm $2a+4b+3c^2=68$
Tìm $\min P=a^2+b^2+c^2$
bài 1 2 ok rồibài 3,4 đi em @@!! –  ღTùngღ 02-09-15 10:12 PM
? có câu dễ mà, –  ๖ۣۜPXM๖ۣۜMinh4212♓ 02-09-15 10:08 PM
$9a^2+1 \geq 6a ;9b^2+4 \geq 12b ;$
=>$9a^2+9b^2+9c^2+5\geq6a+12b+9c^2=3.68=204$
=>$a^2+b^2+c^2\geq\frac{199}9$

Dấu = khi $a=\frac13;b=\frac23;c=\frac{\sqrt{194}}3$
bạn làm nhiều,thử nhiều sẽ quen thôi :D –  ღTùngღ 03-09-15 09:12 AM
tìm điểm rơi ntn v –  tran85295 02-09-15 11:29 PM
ok em yêu ^_^ –  ღTùngღ 02-09-15 11:24 PM
Sử dụng cô - si :
$a^2+4 \geq 4a ;b^2+16 \geq 8b ; \frac{c^3}{2}+\frac{c^3}{2}+32\geq 6c^2$
cộng lại : $a^2+b^2+c^3+52 \geq 4a+8b+6c^2$ mà $2a+4b+3c^2=68$
$=>a^2+b^2+c^3 \geq 84$
c^2 hay c^3 chả đc @@! –  ๖ۣۜPXM๖ۣۜMinh4212♓ 02-09-15 11:27 PM
đm cứ cãi :3 c^3 mà –  ahihi 02-09-15 10:30 PM
đm c^2 mà em yêu -_- –  ღTùngღ 02-09-15 10:28 PM

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