`a) A = 2x^2 - 6x`

`A = 2( x^2 - 2 . 3/2x + 9/4 ) - 9/2`

`A = 2( x - 3/2 )^2 - 9/2`

`Do  2( x - 3/2 )^2 >= 0  AA  x`

`A = 2( x - 3/2 )^2 - 9/2 >= -9/2  AA  x`

`Dấu = xảy  ra  <=> 2( x - 3/2 )^2 = 0`

`x - 3/2 =0`

`x = 3/2`

`Vậy  A min = -9/2 <=> x = 3/2`

`b) B = ( x  - 3 )( x + 5 ) + 4`

`B = x^2 + 2x - 15 + 4`

`B = ( x + 1 )^2 - 12`

`Do  ( x + 1 )^2 >= 0  AA  x`

`B = ( x + 1 )^2 - 12 >= -12  AA  x`

`Dấu = xảy  ra <=> ( x + 1 )^2 =0`

`x + 1 =0 `

`x = -1`

`Vậy  B min = -12 <=> x = -1`

`c) C = x^2 - 6x + 11`

`C = ( x - 3 )^2 + 2`

`Do  ( x - 3 )^2 >= 0  AA  x`

`C = ( x - 3 )^2 + 2 >= 2 AA  x`

`Dấu = xảy  ra <=> ( x - 3 )^2 = 0`

`x - 3 = 0`

`x = 3`

`Vậy  C min = 2 <=> x = 3`

`d) D = 3x^2 - 4x + 10`

`D = 3( x^2 - 2 . 2/3 + 4/9 ) - 4/3 + 10`

`D = 3( x - 2/3 )^2 + 26/3`

`Do  3( x - 2/3 )^2 >= 0  AA  x`

`D = 3( x - 2/3 )^2 + 26/3 >= 26/3 AA  x`

`Dấu = xảy  ra <=> 3( x - 2/3 )^2 = 0`

`x -2/3 =0`

`x = 2/3`

`Vậy  D min = 26/3 <=> x = 2/3`

`e) E = 2x^2 + 5x - 15`

`E = 2( x^2 + 2 . 5/4x + 25/16 ) - 25/8 - 15`

`E = 2( x + 5/4 )^2 - 145/8`

`Do  2( x + 5/4 )^2 >= 0  AA  x`

`E = 2( x + 5/4 )^2 - 145/8 >= -145/8  AA  x`

`Dấu = xảy  ra  <=> 2( x + 5/4)^2 = 0`

`x + 5/4 = 0`

`x = -5/4`

`Vậy  E min = -145/8 <=> x = -5/4`