The origin of sinC and cosC, the second question:

According to (1), tanc = sinc/cosc = √ 5 >; 0

So 0 < c < π/2, sinC=√5cosC.

Compare the above formula with sin? C+cos？ C= 1 Meanwhile,

Sinc = √ 5/√ 6 and COSC =1√ 6 can be obtained.