Solution: let the initial velocity be V0, the final velocity be Vt, the displacement be s, the friction force be f, and m=3kg.

According to the meaning of the question, Vt=0.5V0, S=9m and f=-8N.

It is obtained from the conservation of energy:-fs =1/2mvt2-1/2mv02.

Data brought in: v0 =12m/s.

The math topic is known: tanx=sinx/cosx: sin? x +cos？ x= 1

Then: Tan on the left? x-sin？ X = sin? x/cos？ x - sin？ X= sin? x( 1/cos？ x- 1) = sin？ x( 1 - cos？ x) / cos？ x

= sin? X (sin? x)/cos？ x =tan？ X crime? x