Form5 Amaths show that sinxcosnx=sinnxcosx-sin(n-1)x

如題
show that sinxcosnx=sinnxcosx-sin(n-1)x
唔可以用 減=0既方法 sinxcosnx-sinnxcosx+sin(n-1)x=0
唔可以用 砌數方法 sinxcosnx-cosxsinnx+cosxsinnx=sinnxcosx-sin(n-1)x

即係話 我要 sinxcosnx=...=...=...=sinnxcosx-sin(n-1)x

唔該晒大家!!!