18=a1+2d
-9=a1+11d 27=-9d d=-3 -9=a1+11×(-3) a1=24
a21=24+(n-1)(-3)
a21=24+20×(-3)
a21=-36
a3=18 a12=-9
a1+2d=18
a1+11d=-9
---------------
-9d=27
d=-3
a1-6=18 a1=24
a12=24+20x-3
=24-60
A :-36
a3=18,a12=-9,a21=?
a1+(n-1)*d=an
18+(21-1)*-9
=18-180
=-162
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