=> x2+2x\(\dfrac{3}{2}\)+\(\dfrac{9}{4}\) +\(\dfrac{11}{4}\)
=>\(\left(x^2+2x\dfrac{3}{2}+\dfrac{9}{4}\right)+\dfrac{11}{4}\)
=> \(\left(x+\dfrac{3}{2}\right)^2+\dfrac{11}{4}\)
do \(\left(x+\dfrac{3}{2}\right)^2\ge0\forall x\)
=> \(\left(x+\dfrac{3}{2}\right)^2+\dfrac{11}{4}\ge\dfrac{11}{4}\)
=> GTNN A =\(\dfrac{11}{4}\) khi \(x+\dfrac{3}{2}=0\)
=> x=-\(\dfrac{3}{2}\)
vậy ....
=> -x2+4x
=> -(x2-4x+4-4)
=> -[(x2-4x+4)-4]
=> -[(x-2)2-4]
=> -(x-2)2+4
do -(x-2)2 ≤ 0 ∀x
=> -(x-2)2 +4≤ 4
GTLN B =4 khi x-2=0
=> x=2
vậy .....
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