Question 389888
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Hi,        
Find the number of terms of the sequence 1,4,7,10... for {{{s[n] > 100}}}
{{{S[n]=(n/2)(2*a[1]+ (n-1)d)}}} In this example d = 3 and first term is = 1
    n/2 [2 + 3(n-1)] > 100
     n[2 + 3n-3)] > 200
     2n + 3n^2-3n > 200
    3n^2 - n - 200 = 0  {{{n = (1 +- sqrt( 2401))/(6)= (1+-49)/6}}}n=-8 or n= 8  1/3
{{{S[9]=(4.5)(2 + (8)3)= 117}}}
  Nine terms must be taken for sum to be > 100