Step-by-step explanation:
boy, that is hard to decipher ...
but from what I understood after a while of reading and re-reading, the proof is based on the equal slope (0) of all 3 lines showing that all 3 lines are parallel.
remember, the slope is the ratio y/x indicating how many units y changes, when x changes a certain amount of units when going from one point to another.
so, going from N to M :
x changes by (2d - 2b) units.
y changes by 0 units (2c to 2c).
so, the slope of NM is 0/(2d - 2b) = 0.
going from K to L :
x changes by (2a - 0) = 2a units.
y changes by 0 units (0 to 0).
so, the slope of KL is 0/2a = 0.
R and S are the middle points between K and N, and between L and M.
R = ((2b+0)/2, (2c+0)/2) = (b, c)
S = ((2d + 2a)/2, (2c + 0)/2) = ((d+a), c)
so, going from R to S :
x changes by (d+a-b) units.
y changes by 0 units (c to c).
so, the slope of KL is 0/(d+a-b) = 0.
as all 3 slopes are equal (and with being 0 also parallel to the x-axis), this proves that the line RS is parallel to its bases.
