Know Tana, find Sina, and use the basic relationship of trigonometric function with the same angle;

1+(tana)^2=(seca)^2= 1/(cosa)^2= 1/[ 1-(sina)^2]

In this problem, because tana = 3 >；; 0, a is the inner angle of a triangle, so a is an acute angle,

The trigonometric function values of a are all positive, so sinA is found.