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(28)
an=a1+(n-1)d
a4=5
a1+3d=5 (1)
a7=11
a1+6d=11 (2)
(2)-(1)
3d=6
d=2
from (1)
a1+6=5
a1=-1
an =-1+2(n-1) = 2n-3
S6 = 3(a1+a6) = 3(-1+9) =24
an=a1+(n-1)d
a4=5
a1+3d=5 (1)
a7=11
a1+6d=11 (2)
(2)-(1)
3d=6
d=2
from (1)
a1+6=5
a1=-1
an =-1+2(n-1) = 2n-3
S6 = 3(a1+a6) = 3(-1+9) =24
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