[Toán 9] $maxS=ab+2bc+3ac$

S

son9701

Ta có:
$S= a(b+c)+2c(a+b)= a(1-a)+2c(1-c)=a-a^2+2c-2c^2=-(a-\frac{1}{2})^2-2(c-\frac{1}{2})^2+\frac{3}{4}$ \leq $\frac{3}{4}$

S= 0,75 khi a=c=0,5;b=0
 
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