Answer:
Segment DE is half the length of segment AC. -- Substitution property of equality
Step-by-step explanation:
Here is given a proof for proving line joining mid segment is parallel and half the length of third side.
Stepwise proof is given in two columns
We find that for every line there is a justification as
STatement Justification
D and E coordinates found out MId point formula
DE and BC are measured Distance formula
DE=1/2 BC Substitution property of equality
This justification was missing in the given proof and with this included proof would be complete
Answer:
Part a)
Part b) 
Part c) (s+t) lie on Quadrant IV
Step-by-step explanation:
[Part a) Find sin(s+t)
we know that

step 1
Find sin(s)

we have

substitute




---> is positive because s lie on II Quadrant
step 2
Find cos(t)

we have

substitute




is negative because t lie on II Quadrant
step 3
Find sin(s+t)

we have



substitute the values



Part b) Find tan(s+t)
we know that
tex]tan(s + t) = (tan(s) + tan(t))/(1 - tan(s)tan(t))[/tex]
we have



step 1
Find tan(s)

substitute

step 2
Find tan(t)

substitute

step 3
Find tan(s+t)

substitute the values




Part c) Quadrant of s+t
we know that
----> (s+t) could be in III or IV quadrant
----> (s+t) could be in III or IV quadrant
Find the value of cos(s+t)

we have



substitute



we have that
-----> (s+t) could be in I or IV quadrant
----> (s+t) could be in III or IV quadrant
----> (s+t) could be in III or IV quadrant
therefore
(s+t) lie on Quadrant IV
- <F=180-106+29=180-135=45°
In both triangles
Hence both triangles are similar by AA congruency
Answer:

Step-by-step explanation:
Given
Points: (-5,0) and (-4, y)
Slope:
Required
Find y
The slope (m) of a line is calculated using:

So, we have:





(1, 3) are the correct coordinates of the intersection (or solution) of this system.