1.

11b) Find the sum of 20 terms of the A.P. 12, 15, 18---​

Answer»

Answer:

810

Step-by-step EXPLANATION:

Here, a=12, d=15-12=3, n =20.

now, S=n/2(2a+(n-1)d)

           =20/2(2×12+(20-1)3)

            =10(24+57)

             =810.



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