Đáp án+Giải thích các bước giải:

`C=12-x-x^2`

`C=49/4-1/4-x-x^2`

`C=-(x^2+2.x. 1/2+1/4)+49/4`

`C=-(x+1/2)^2+49/4`

Ta có:

`-(x+1/2)^2≤0∀x∈R`

`⇒-(x+1/2)^2+49/4≤49/4∀x∈R`

`C_{min}=49/4⇔x+1/2=0⇔x=-1/2`

Vậy `C_{min}=49/4⇔x=-1/2`

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`D=15-3x-2x^2`

`D=249/16-9/16-3x-2x^2`

`D=-2(x^2+2.x. 3/4+9/16)+249/16`

`D=-2(x+3/4)^2+249/16`

Ta có:

`-2(x+3/4)^2≤0∀x∈R`

`⇒-2(x+3/4)^2+249/16≤249/16∀x∈R`

`D_{min}=249/16⇔x+3/4=0⇔x=-3/4`

Vậy `D_{min}=249/16⇔x=-3/4`