Trong các mệnh đề sau, mệnh đề nào là mệnh đề sai:

\[{\rm{sin}}\left( {{\rm{2a}}} \right){\rm{ = 2sin}}\left( {\rm{a}} \right){\rm{cos}}\left( {\rm{a}} \right)\]

\[{\rm{sin}}\left( {{\rm{2a}}} \right){\rm{ = sin}}\left( {\rm{a}} \right){\rm{cos}}\left( {\rm{a}} \right)\]

\[{\rm{cos}}\left( {{\rm{2a}}} \right){\rm{ = co}}{{\rm{s}}^{\rm{2}}}\left( {\rm{a}} \right) - {\rm{si}}{{\rm{n}}^{\rm{2}}}\left( {\rm{a}} \right)\]

\[{\rm{cos}}\left( {{\rm{2a}}} \right){\rm{ = 1}} - {\rm{2si}}{{\rm{n}}^{\rm{2}}}\left( {\rm{a}} \right)\]