10*20*30*40*-----*1000?
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This is arithmetic progression - each member after the first one is previous one plus the same number(in this case d=10).
a(1)=10, a(n)=1000, n=100, d=10
a(2)=a(1)+d, a(3)=a(2)+d=a(1)+d+d= a(1)+(n-1)*d
a(n)=a(1)+(n-1)*d; a(100)=10+99*10
Sum = {[a(1)+a(n)]/2}*n
Sum={[a(1)+a(100)]/2}*100
Sum={[10+1000]/2}*100
Sum=50500
a(1)=10, a(n)=1000, n=100, d=10
a(2)=a(1)+d, a(3)=a(2)+d=a(1)+d+d= a(1)+(n-1)*d
a(n)=a(1)+(n-1)*d; a(100)=10+99*10
Sum = {[a(1)+a(n)]/2}*n
Sum={[a(1)+a(100)]/2}*100
Sum={[10+1000]/2}*100
Sum=50500
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