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Step-by-step explanation:
3rd term is 7
general term is a + (n-1)d
7=a+2d
7th term is:
a+6d
2+(a+6d)3=7
(a+6d)3=5
3a+18d=5
3a+6d=21
12d=-16
d=-4/3
a+2(-4/3)=7
a=7+8/3
a=29/3
sequence is 29/3 , 25/3 , 7 ,17/3...........
sum of 20 terms is n/2*(2a+(n-1)d)
10*(2*29/3+19*-4/3)
10*(58/3-76/3)
10*(-18/3)
10*-6
-60
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