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r=2-5
r=-3
a1=5-(-3)
a1=8
an=a1+(n-1)*r
an=8-3n+3
an=-3n+11
a5=-3*5+11=-15+11
a5= -4
a10=-3*10+11 =-30+11
a10=-29
Sn=(a1+an)*n/2
S10=(8-29)*10/2 =-21*5
S10 = -105