Question 923402
Arithmetic Sequence: d = 5
{{{a[n] = a[1] + (n-1)d}}}
{{{a[100] = 3 + (99)5}}} = 498  Step I
.......

{{{S[n]=(n(a[1]+a[n]))/2}}}
{{{S[100]=50(3 + 498)}}}  Step 2
.......
Re TY
complete the Calculation to find sum = 25,050
{{{S[100]=50(3 + 498)}}} = 25,050  Step 3
this is the way one would find the Sum regardless of the Number of terms.