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r=1/3
an=a1+(n-1)*r
a10=-2+9*1/3=-2+3=1
Sn=(a1+an)*n/2
S10=(-2+1)*10/2= -1*5=-5
r = 1/3
a₁₀ = a₁+9r = -2+3 =1
S₁₀ = (a₁+a₁₀)/2 *n = (-2+1)/2 *10 = -5