* 8th term of an arithmetic sequence is 21.
a) What is the sum of its 6th term and 10th term?
b) What is the sum of 1st term and 15th term?
c) calculate the sum of first 15 term
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Answer:
Given,
a
1
+a
20
=88
a.
a
2
+a
19
=(a
1
+d)+(a
20
−d)
=a
1
+a
20
=88
b.
a
10
+a
11
=a
1
+9d+a
20
−9d
42+a
11
=a
1
+a
20
∴a
11
=88−42=46
c.
d=a
11
−a
10
=46−42=4
d.
a
10
=a+9d
a=42−9(4)
∴a=6
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