数列　 2|4 6|8 10 12|14 16 18 20| ... に対して h=1223334444 は何番目(n)のグループの何項(m)の数になるか．

Clear[b]

b[n_] := 2 + 2*Sum[k, {k, 1, n - 1}]

b[1]

Table[b[n], {n, 1, 6}]

b[n]

Clear[a]

a[n_, m_] := b[n] + 2 (m - 1)

a[2, 2]

a[3, 3]

Clear[data]

data[n_] := Table[a[k, l], {k, 1, n}, {l, 1, k}]

data[3]

TableForm[data[4]]

Clear[h]

h = 1223334444

Clear[n]

NSolve[b[x] == h]

x   /. NSolve[b[x] == h]

Max[x  /. NSolve[b[x] == h]]

n = Floor[Max[x /. NSolve[b[x] == h]]]

Solve[a[n, y] == h, y]

y /. Solve[a[n, y] == h, y][[1]]

Clear[m]

m = y /. Solve[a[n, y] == h, y][[1]]

a[n, m]