Question

Decide whether following sequence is an A.P., if so find the 20th term of the
progression.
-12, -5, 2, 9, 16, 23, 30, ...
​

Answer 1

Answer:

Step-by-step explanation:

We see that the difference is 7.

9 - 2 = 7 ; 2 - ( -5) = 7

a = - 12 which is the first term.

n = 20

S$n$ = ( 2a + ( n - 1 )d )

Therefore ,

S$20$ = ( 2 * -12 + ( 20 - 1 ) 7 )

      = - 24 + 19 * 7

      =  109

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