giải phương trình mũ : a) (3+ √5)^{x} + (7- √5) ^{x} = 8 b) 3.5 ^{2x-1} - 2.5 ^{x-1} = 0,2

$$\eqalign{ & b)\,\,{3.5^{2x - 1}} - {2.5^{x - 1}} = 0,2 \cr & \Leftrightarrow {3.5.5^{2x - 2}} - {2.5^{x - 1}} = 0,2 \cr & Dat\,\,t = {5^{x - 1}}\,\,\left( {t > 0} \right) \Rightarrow {t^2} = {5^{2x - 2}} \cr & PT:\,\,15{t^2} - 2t - 0,2 = 0 \cr & \Leftrightarrow \left[ \matrix{ t = {1 \over 5}\,\,\left( {tm} \right) \hfill \cr t = - {1 \over {15}}\,\,\left( {ktm} \right) \hfill \cr} \right. \cr & \Leftrightarrow {5^{x - 1}} = {1 \over 5} = {5^{ - 1}} \Leftrightarrow x - 1 = - 1 \Leftrightarrow x = 0 \cr} $$