Find the sum of the arithmetic progression of 11, 5,-7,-1
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Answer:
a=11,d=5-11= -6, Sn=?, n=?,an= -1
an=a+(n-1)d
-1= 11(n-1)-6
-11-1= -6n+6
-12-6= -6n
-18= -6n
n= -18/-6
n=3
Sn=2a+(n-1)d
=22+2×-6
=22-12
=10
Sn=10
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