B is an Obtuse angle! and I’m 100% sure!
Answer: 18
Step-by-step explanation: The formula for circumference is 2piR. Since 2piR = 36pi, you can cancel out the pi's which makes it 2r = 36. Divide by 2 on both sides and you get r = 18.
Let the first term, common difference and number of terms of an AP are a, d and n respectively.
Given that, 9th term of an AP, T9 = 0 [∵ nth term of an AP, Tn = a + (n-1)d]
⇒ a + (9-1)d = 0
⇒ a + 8d = 0 ⇒ a = -8d ...(i)
Now, its 19th term , T19 = a + (19-1)d
= - 8d + 18d [from Eq.(i)]
= 10d ...(ii)
and its 29th term, T29 = a+(29-1)d
= -8d + 28d [from Eq.(i)]
= 20d = 2 × T19
Hence, its 29th term is twice its 19th term
