A=x²+6x+12
   =x²+2.x.3+9+3
   =(x+3)²+3
Ta có (x+3)²≥0 ⇒(x+3)²+3 ≥3
Dấu "=" xảy ra khi: 
x+3=0  =>x=-3
Vậy MinA=3 khi x=-3
B=x²-10x-5
   =x²-2.x.5+25-30
   =(x-5)²-30
Ta có (x-5)²≥ 0 ⇒ (x-5)²-30≥ -30
Dấu "=" xảy ra khi:\
x-5=0 ⇒x=5
Vậy MinB=-30 khi x=5
C=4x²-3x-2
   =(2x)²-2.2x.3/4+9/16-17/16
   =(2x-3/4)²-17/16
Ta có: (2x-3/4)² ≥0  ⇒ (2x-3/4)²-17/16≥-17/16
Dấu "=" xảy ra khi:
2x-3/4=0 ⇒2x=3/4 ⇒x=3/8
Vậy MinC=-17/16 khi x=3/8
D=x²-5x+4
   =x²-2.x.5/2+25/4-9/4
   =(x-5/2)²-9/4
Ta có (x-5/2)²≥0 ⇒(x-5/2)²-9/4≥-9/4
Dấu "=" xảy ra khi:
x-5/2=0 ⇒x=5/2
Vậy MinD=-9/4 khi x=5/2
E=9x²-6x-1000
   =(3x)²-2.3x+1-1001
   =(3x-1)²-1001
Ta có: (3x-1)²≥0⇒ (3x-1)²-1001≥-1001
Dấu "=" xảy ra khi:
3x-1=0⇒3x=1⇒x=1/3
Vậy MinE=-1001 khi x=1/3
F=4x²-3x-2
   =(2x)²-2.2x.3/4+9/16-41/16
   =(2x-3/4)²-41/16
Ta có (2x-3/4)²≥ 0 ⇒(2x-3/4)²-41/16≥-41/16
Dấu "=" xảy ra khi:
2x-3/4=0  ⇒2x=3/4  ⇒x=3/8
Vậy MinF=-41/16 khi x=3/8
G=2x²+4x+2021
   =2x²+4x+2+2019
   =2(x²+2x+1)+2019
   =2(x+1)²+2019
Ta có: 2(x+1)²≥0 ⇒  2(x+1)²=+2019≥2019
Dấu "=" xảy ra khi:
x+1=0  ⇒x=-1
Vậy MinG=2019 khi x=-1
H=2x²+3x+2021
 ⇒2H=4x²+6x+4042
         =(2x)²+2.2x.3/2+9/4-16159/4
         =(2x+3/2)²-16159/4
Ta có  (2x+3/2)²≥0 ⇒  (2x+3/2)²-16159/4≥-16159/4
Dấu "=" xảy ra khi:
2x+3/2=0 ⇒2x=-3/2⇒x=-3/4
⇒MinH=-16159/4:2=-16159/8
Vậy MinH=-16159/8 khi x=-3/4