1.

Find the sum of the first 18 terms of the Ap 11,8,5,3...​

Answer»

GIVEN :

AP : 11,8,5,3...

Difference = A2 - a1 = 8 - 11 = -3

In an AP,

SN = n/2 [ 2A + (n-1)d ]

= 18/2 [ 2(11) + (18-1)-3 ]

= 9 [ 22 + (17)-3 ]

= 9 [ 22 + (-51) ]

= 9 [ 22 - 51 ]

= 9 [ -29]

= -261

Therefore, the sum of first 12 terms is -261



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