Question

16. The 3

rd term of an AP is 8 and the 9

th term of the AP exceeds three times

the 3

rd term by 2. Find the sum of its first 21 terms​

Answer 1

Answer:

S21=672

Step-by-step explanation:

T3=a+2d=8---------(1)

and T9=3*T3+2

T9=3*8+2=26

Or a+8d=26.............(2)

Subtracting (1) from(2)

6d=18, d=3

and from(1) putting d=3

a+6=8, a=2

Sum of first 21 terms

S21=21/2( 2*2+20*3)

=21/2(64)=21*32=672

Answer 2

Answer:

S21= 672

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