=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 .