Question 634655
  <pre><font face = "Tohoma" size = 3 color = "indigo"><b> 
Hi,
Re TY, one may use either of the 2 formulas*** to find the sum
3, 7, 11, 15, 19...,   Arithmetic sequence, d = 4
For sums of an arithmetic sequence one can use:
***{{{S[n]=(n/2)(2*a[1]+ (n-1)d)}}}  OR
***{{{S[n] = n(a[1] + a[n])/2}}}

{{{S[50] = 50( 3 + a[50])/2}}}  |Using the Latter after finding {{{a[50]}}}
{{{a[n] = a[1] + (n-1)d}}}  &#8658;   {{{a[50] = 3 + 49*4 = 199}}} 
{{{S[50] = highlight(50)*(3 + 199)/2 = 50*101 = 5050}}}