giúp mình câu b với ạ

`a)`

Do `0 < alpha < pi/2` `=>` `cosalpha = sqrt(1-sin^2alpha)=(sqrt6)/3`

Ta có:

`sin2alpha=2sinalphacosalpha=(2sqrt2)/3`

`cos2alpha=1-2sin^2alpha=1/3`

`=>` `tan2alpha=2sqrt2`

`=>` `cot2alpha=sqrt2/4`

`b)`

`sin \ alpha/2 = 3/4`

`=>` `sin^2 \ alpha/2 = 9/16`

`=>` `(1-cosalpha)/2 = 9/16`

`=>` `1-cosalpha=9/8`

`=>` `cosalpha=-1/8`

Do `pi < alpha < 2pi` `=>` `sinalpha=-sqrt(1-cos^2alpha)=-(3sqrt7)/8`

Ta có:

`sin2alpha=2sinalphacosalpha=(3sqrt7)/32`

`cos2alpha=1-2sin^2alpha=-31/32`

`=>` `tan2alpha=-(3sqrt7)/31`

`=>` `cot2alpha=-(31sqrt7)/21`