a5=3

a2+a4+a6+a8+a 10=5a6=30

a6=6

d=3

2 Sn =-N2 S(n- 1)=-(n- 1)2 =-N2+2n- 1

sn-S(n- 1)= an =-N2+N2-2n+ 1 =-2n+ 1

d=-2

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