[toán 12] bt

N

nhox_lan

Bài 1:

$2^x + 4^x = 20$

\Leftrightarrow $2^x + (2^2)^x = 20$

\Leftrightarrow $2^x + (2^x)^2 = 20$

Đặt t = $2^x$ ( Đk: t\geq0 )

Pt \Leftrightarrow $ t^2 + t - 20 = 0$

\Leftrightarrow $\left[\begin{matrix}t = 4(tm)\\ t = -5 (loại)\end{matrix}\right.$

\Rightarrow $2^x = 4$

\Rightarrow $x = 2$
 
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