Question

Find the sum of following series, 0.9,0.91,0.92,.......100 terms

Answer 1

$S_{100}=\frac{100}{2}(2(0.9)+(100-1)(0.01))=139.5$

Answer 2

S₁₀₀=n/2(2a+(n-1)d)
       =100/2(2*0.9+(100-1)0.01) 
       =50(1.8+0.99)
       =50*2.79
         =139.5