Answer:
Proven.
Explanation:
So, fasten your seatbelts, we're about to go on a long ride(◠‿・)—☆
» In a parallelogram, opposite sides are equal in length.
This can be represented (in the vector way) by
'QR = '-SP = 'PS and 'RS = 'QS (let “'” represent a vector)
'QR + 'SP= 0 and 'RS-'QS = 0
Note the signs. The signs are determined by the arrows in the diagram. [A reminder: If you walk from a point A to a point B, your displacement vector is 'AB. If you walk back, that is, from B to A; your displacement vector is 'BA which is also equal to '-AB.]
This means: to prove that PQRS is a parallelogram, we must prove that
'QR + 'SP= 0 and/or 'RS-'QS = 0.
In this solution, we will only work on the first equation.
Permit me to remove this ' sign.
QR = QC+CR ...1
QR= QB+BA+AD+DR...2
ADD 1 TO 2
2QR= QC+CR+QB+BA+AD+DR
[LOOK CLOSELY, QC+QB=0 ; CR+DR=0. This is because BQ+QC = BC
BUT, BQ=QC= (BC)/2
QC-BQ= 0
AND, -BQ= QB
QC+QB= 0 ;)]
2QR= BA+AD...3
★
SP = SA+AP...4
SP= SD+DC+CB+BP...5
ADD 4 TO 5
2SP = SA+AP+SD+DC+CB+BP
[AGAIN, SA+ SD = 0 ; AP+ BP = 0]
2SP= DC+CB...6
3+6
2QR+2SP= BA+AD+DC+CB
[ BA+AD+DC+CB, IN A SIMPLE WAY, MEANS:
you start from B, and move to A (BA); from A to D (AD); from D to C (DC); and from C to B (CB). Therefore, you have gone from B to B (from your starting point and back). Hence, your total DISPLACEMENT is zero (you haven't changed your position).
2(QR+SP) = 0
'QR+'SP= 0
PROVEN
You can work out the other equation, but it is evident that PQRS is a parallelogram.