theorem Th51:
for
t,
r1,
r2 being
Real st
r1 <= r2 holds
(
t in [.r1,r2.] iff ex
s1 being
Real st
(
0 <= s1 &
s1 <= 1 &
t = (s1 * r1) + ((1 - s1) * r2) ) )
theorem Th87:
for
r1,
r2,
s1,
s2 being
Real st
r1 < r2 &
s1 < s2 holds
[.r1,r2,s1,s2.] is
Jordan