u(x) = anxn , (1.82)
n=0
or by using few terms of the series
u(x) = a + a x + a x2 + a x3 + a x4 + a x5 + a x6 + · · · (1.83)
0 1 2 3 4 5 6
Differentiating term by term gives
u (x) = a + 2a x + 3a x2 + 4a x3 + 5a x4 + 6a x5 + · · ·
1 2 3 4 5 6
u (x) = 2a + 6a x + 12a x2 + 20a x3 + 30a x4 + · · · (1.84)
2 3 4 5 6
u (x) = 6a + 24a x + 60a x2 + 120a x3 + · · ·
3 4 5 6