3.Sum of N Terms of Arithmetic Progression

Sum of series 

S= n/2{2a+(n-1)d}

Find sum of series for 

3,6,9,12....... Up to 20 terms 

first term 3

common difference 6-3=3

number of terms n=20 

S = 20/2 {2×3+(20-1)×3}

    = 10 {6+19×3}

   = 10 × {6+57}

    = 630