in an arithmetic progression if n is equal to 2 and minus 1 then what is Sn
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n=2, -1
Sn = 2/2(2a+(2-1)d)
= 2a - d
Sn = -1/2(2a+(-1-1)d)
= -1/2(2a -2d)
= -a + d
therefore,
-a +d = 2a -d
2d = 3a
a = 2d/3
-2d/3 + d = Sn
-2d + 3d/3 = Sn
d/3 = Sn
Sn = 2/2(2a+(2-1)d)
= 2a - d
Sn = -1/2(2a+(-1-1)d)
= -1/2(2a -2d)
= -a + d
therefore,
-a +d = 2a -d
2d = 3a
a = 2d/3
-2d/3 + d = Sn
-2d + 3d/3 = Sn
d/3 = Sn
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