Câu `4:`

`A.` `cos60=sin30=1/2` `->` đúng 

`B.` `cos60=1/2`

`sin120=sqrt3 /2`

`->cos60 ne sin120`

`->` Sai

`C.` `cos30=sin120=sqrt3 /2` `->` đúng

`D.` `sin60=sqrt3 /2`

`-cos120=-(-1/2)=1/2`

`->sin60 ne -cos120`

`->` Sai