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A3=7
a+2d=7-------(1) equation
A7=3×a3+2
a+6d =3×7+2
=21+2
a+6d =23-----(2) equation
from 1and 2equation...
a+2d=7
a+6d=23
- - -
________
-4d=-16
d=6
a+2d=7
a+2×6=7
a=-5
s20=n/2[2a+(n-1)d]
=20/2[2×-5+(20-1)6]
10(-10+19×6)
10(-10+144)
10×104
1040
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