`a)` Ta có:
`=>0<cos35°<1`
`=>1/{cos35°}>1`
`=>{sin 35° }/{cos35°}>{sin35°}/1`
`=>tan35°>sin35°`
Vậy: `tan35°> sin35°`
$\\$
`b)` Tính chất: `sin^2α+cos^2α=1`
Vì `sinα=0,75=3/4`
`=>cos^2α=1-sin^2α=1-(3/4)^2`
`=1-9/{16}=7/{16}`
`=>`$\left[\begin{array}{l}cosα=\sqrt{\dfrac{7}{16}}=\dfrac{\sqrt{7}}{4}\\cosα=-\sqrt{\dfrac{7}{16}}=\dfrac{-\sqrt{7}}{4}\end{array}\right.$
Vậy: `cosα=\sqrt{7}/4` hoặc `cosα={-\sqrt{7}}/4`
