Error

f= f´=
n                    r1 r2
───   ─────────────
n-1     e(n - 1) - n r1 + n r2
1
──  = (n-1)
(
f
1
──  -
r1
1
──  +
r2
(n-1) e
────
nr1   r2
)
f         n         e         r1         r2
h = -
f (n - 1) e
──────
n r2
h = -
f (n - 1) e
──────
n r2
h o (-h´)      f           n           e       r2 o r1
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