Theorem: the segment joining the midpoints of two sides of a triangle is parallel to the third side and half its length.
a two column proof of the theorem is shown, but a justification is missing.

a triangle with vertices a at 6, 8. b is at 2, 2. c is at 8, 4. segment de. point d is on side ab and point e is on side bc

statement justification
the coordinates of point d are (4, 5) and coordinates of point e are (5, 3) midpoint formula
length of segment de is square root of 5 and length of segment ac is 2 multiplied by the square root of 5. distance formula
segment de is half the length of segment ac.
slope of segment de is −2 and slope of segment ac is −2. slope formula
segment de is parallel to segment ac. slopes of parallel lines are equal.

which is the missing justification?
additive identity
by construction
midsegment theorem
substitution property of equality

Aprisoner is trapped in a cell containing three doors. the first door leads to a tunnel that returns him to his cell after two days of travel. the second leads to a tunnel that returns him to his cell after three days of travel. the third door leads immediately to freedom. (a) assuming that the prisoner will always select doors 1, 2 and 3 with probabili- ties 0.5,0.3,0.2 (respectively), what is the expected number of days until he reaches freedom? (b) assuming that the prisoner is always equally likely to choose among those doors that he has not used, what is the expected number of days until he reaches freedom? (in this version, if the prisoner initially tries door 1, for example, then when he returns to the cell, he will now select only from doors 2 and 3.) (c) for parts (a) and (b), find the variance of the number of days until the prisoner reaches freedom. hint for part (b): define ni to be the number of additional days the prisoner spends after initially choosing door i and returning to his cell.