a. tanα=1/3 => (sinα/cosα)=1/3=>sinα=1/3.cosα
Ta co: (sinα)^2+(cosα)^2=1 <=> 1/9.(cosα)^2+(cosα)^2=1 => cosα=3/sqrt(10)
=> sinα=1/sqrt(10)
b. cotα = 3/4 => (cosα/sinα)=3/4 => cosα=3/4.sinα
Ta co: (sinα)^2+(cosα)^2=1 <=> (sinα)^2+9/16.(sinα)^2=1 => sinα=4/5
=> cosα=3/5