i) The sum of the 3rd and 7th terms of an A.P. is 32 and their product is 220. Find
the sum of first twenty-one terms of the A.P. (Take the value of d positive).
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Step-by-step explanation:
as a=16-4d
=16-4×3
=16-12
=4
*a=4
which is substituted in the sn formula
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