Có: `sin^2x+cos^2x=1`
`<=> (3/4)^2+cos^2x=1`
`<=> 9/16+cos^2x=1`
`<=> cos^2x=7/16`
`=> cosx=\sqrt{7}/4` (do `0<x<π/2`)
Lại có: `tan x=(sinx)/(cosx)=(3/4)/(\sqrt{7}/4)=3/\sqrt{7}=(3\sqrt{7})/7`
`cot x=1/(tanx)=\sqrt{7}/3`
Vậy `cosx=\sqrt{7}/4; tanx=(3\sqrt{7})/7; cotx=\sqrt{7}/3` khi `sin x=3/4` (`0<x<π/2`)