Câu 4
$A=|2x-3|+14$
Ta có: $|2x-3|≥0∀x$
$⇒|2x-3|+14≥14∀x$
Hay $A≥14∀x$
Dấu = xảy ra $⇔2x-3=0⇔x=\dfrac{3}{2}$
Vậy $Min_A=14$ tại $x=\dfrac{3}{2}$
Câu 5
Ta có: `(3x+9)^2≥0∀x`
`⇒-2.(3x+9)^2≤0∀x`
`⇒-2.(3x+9)^2-12≤-12∀x`
Hay `B≤-12∀x`
Dấu $=$ xảy ra $⇔3x+9=0⇔x=-3$
Câu 6
$5^n.5+\dfrac{1}{5}.5^n=26.5^6$
$⇔5^n(5+\dfrac{1}{5})=26.5^6$
$⇔5^n.\dfrac{26}{5}=26.5^6$
$⇔5^n.\dfrac{1}{5}.26=26.5^6$
$⇔5^n.\dfrac{1}{5}=5^6$
$⇔5^n=5^6:\dfrac{1}{5}=5^7$
$⇔n=7(t/m)$
