a: \(cos55^0=sin\left(90^0-55^0\right)=sin35^0\)
\(cos65^0=sin\left(90^0-65^0\right)=sin25^0\)
Vì 25<35<36<42
nên \(sin25^0< sin35^0< sin36^0< sin42^0\)
=>\(cos65^0< cos55^0< sin36^0< sin42^0\)
b: \(cot77^0=tan\left(90^0-77^0\right)=tan13^0;cot65^0=tan\left(90^0-65^0\right)=tan25^0\)
Vì 13<18<25<57
nên \(tan13^0< tan18^0< tan25^0< tan57^0\)
=>\(cot77^0< tan18^0< cot65^0< tan57^0\)
c: \(cos70^0=sin\left(90^0-70^0\right)=sin20^0;cos29^0=sin\left(90^0-29^0\right)=sin61^0\)
Ta có: 20<50<61
=>\(sin20^0< sin50^0< sin61^0\)
=>\(cos70^0< sin50^0< cos29^0< 1\)
Vì 72>45
nên tan72>1
=>cos70<sin50<cos29<tan72
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