=0。

Sin30 degrees = 1/2.

Cos pie =-cos0

? =- 1。

Three.

1。 Because? Arithmetic progression: an = a1+(n-1) d.

Sn=n(a 1+an)/2

? So what? According to the meaning, 5=a 1+(n- 1)x2.

0=n(a 1+5)/2

? By solving this equation set about a 1 n, we get:

a 1=-5，n=6 .