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2014-10-12T11:28:04+03:00
A3=a1+2r
3+2r=7
2r=4 ⇒r=2

S₁₀= \frac{(a1+a10) *10}{2} =(a1+a1+9r)*5=(2a1+9r)*5=(2*3+9*2)*5=(6+18)*5=24*5=120
2 3 2