13th term of an A. P is 3 and the sun of first 13 terms is 234. Find Common differenceandand the sum of first 21 term
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Step-by-step explanation:
a+12d=3
13(2a+12d)/2=234
a+(a+12d)=234×2/13
a+3=36
a=33
12d=3-33=-30
d=30/12=10/4=-5/2
S21=21(66+(20×-5/2))/2
= 21(66-50)/2
= 21×16/2
= 21×8
=168
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