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a, 4 - x² +2x = -(x²-2x+1)+3

                    =-(x-1)² +3

vì -(x-1)² <= 0 vs mọi x =>-(x-1)² +3 <=vs mọi x 

=>-(x-1)² +3 <= 3

dâu ''='' xay ra khi va chi khi x-1=0 =>x=1

vay ....

b,4x - x² = -(x²-4x+4)-4

              =-(x -2)² -4

 vi -(x-2)²<=0 vs mọi x suy ra -(x-2)² -4 <=0

=>-(x-2)² -4 <=-4

dau = xay ra khi va chi khi x-2=0 =>x=2

vậy......

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