Answer:
- 18
Step-by-step explanation:
arithmetic sequences are in form:
a(n)=a(o)+d(n-1)
sum are in form:
s(n)=[a(0)+a(n)](n/2)
three middle terms is:
10th, 11th, 12th term
How to think: 21 = 10+1+10 --> middle 3 is 10,11,12
sum of these 3 terms:
a(10)+a(11)+a(12)
=a(o)+9d+a(0)+10d+a(0)+11d
=3a(0)+30d=126
a(0)+10d=42 ......(equation 1)
Last three terms is:
19th, 20th, 21th
a(19)+a(20)+a(21)
=a(0)+18d+a(0)+19d+a(0)+20d
=3a(0)+57d=288
a(0)+19d=96 ......(equation 2)
now you have
a(0)+10d=42 ......(equation 1)
a(0)+19d=96 ......(equation 2)
solve and gives
d=6
a(0)= -18
