sns


@edward_
write
cot (89r) as tan (90(89r))
which is tan (r+1)
now add 1 to each term,
so you have
$t_r= tan (r) tan (r+1) + 1 1$
now use tan ((r+1)(r))
@edward_
write
cot (89r) as tan (90(89r))
which is tan (r+1)
now add 1 to each term,
so you have
now use tan ((r+1)(r))