Answer: I think it is -0.4
Step-by-step explanation:
For sake of convenience, reverse the series, then it would be:
21,19,17.....7
a=21, d = -2,
s(4) = 4/2 (21+15)
s(4)=2(36)
s(4) = 72
<span>arc length = circumference • [central angle (degrees) ÷ 360]
Solving this equation for circumference:
</span>
<span>circumference = arc length / (central angle / 360)
</span><span>circumference = 12 / (85/360)
</span>circumference = 12 / <span><span>0.2361111111
</span>
</span>
<span>circumference =
</span>
<span>
<span>
<span>
50.8235294118
</span>
</span>
</span>
Source:
http://www.1728.org/radians.htm
