Tìm y, biết :
\(\eqalign{ & a)y + 45\% .y = – 1,45 \cr & b)y – 25\% .y = {1 \over 2}. \cr} \)
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\(\eqalign{ & a)y + 45\% .y = – 1,45 \cr & y + {{45} \over {100}}y = {{ – 145} \over {100}} \cr & y + {9 \over {20}}y = {{ – 29} \over {20}} \cr & \left( {1 + {9 \over {20}}} \right)y = {{ – 29} \over {20}} \cr & \left( {{{20} \over {20}} + {9 \over {20}}} \right)y = {{ – 29} \over {20}} \cr & {{29} \over {20}}y = {{ – 29} \over {20}} \cr & y = {{ – 29} \over {20}}:{{29} \over {20}} \Leftrightarrow y = – 1. \cr & b)y – 25\% y = {1 \over 2} \cr & y – {{25} \over {100}}y = {1 \over 2} \cr & y – {1 \over 4}y = {1 \over 2} \cr & \left( {1 – {1 \over 4}} \right)y = {1 \over 2} \Leftrightarrow \left( {{4 \over 4} – {1 \over 4}} \right)y = {1 \over 2} \cr & {3 \over 4}y = {1 \over 2} \Leftrightarrow y = {1 \over 2}:{3 \over 4} \cr & y = {1 \over 2}.{4 \over 3} \Leftrightarrow y = {2 \over 3} \cr} \)