Có: `sin^2a+cos^2a=1`
`<=> sin^2a+1/16=1`
`<=> sin^2a=15/16`
`<=> sina=+-\sqrt{15}/4`
`-> tana=(sina)/(cosa)=(+-\sqrt{15}/4)/(1/4)=+-\sqrt{15}`
`\qquad cota=1/(tana)=+-1/\sqrt{15}=+-\sqrt{15}/15`
Vậy `sina=+-\sqrt{15}/4; tana=+-\sqrt{15}; cot a=+-\sqrt{15}/15`