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$\text{a)Ta có:}$

$\text{15 = 3 · 5}$

$\text{27 = 3³}$

$\text{⇒BCNN(15,27) = 3³ · 5 = 135}$

$\text{b)Vì 12 $\vdots$ x , 36 $\vdots$ x , 60 $\vdots$ x}$

$\text{⇒x ∈ ƯC(12,36,60)}$

$\text{Ta có:}$

$\text{12 = 2² · 3}$

$\text{36 = 2² · 3²}$

$\text{60 = 2² · 3 · 5}$

$\text{⇒ƯCLN(12,36,60) = 2² · 3 = 12}$

$\text{⇒x ∈ ƯC(12,36,60) = Ư(12) = {±1;±2;±3;±4;±6;±12} }$

$\text{Mà x ≥ 6}$

$\text{⇒x ∈ {6;12} }$

$\text{Vậy x = 6 hoặc x = 12}$