August 31st, 2018, 09:55 AM  #1 
Member Joined: Aug 2018 From: Nigeria Posts: 73 Thanks: 2  Help me again: what went wrong
Hello everyone... It's me again...The young student who you always help and thanks to you all I'm improving in math calculations and reasoning ability.... Thank you all. I would like to give thanks to "skipjack"[by username] who has always been there for me. ...and referring to my last post that post help me a lot... and I really appreciate everyone who contributed to the post it really help me... Now, I have another question if you won't mind... The equations of sides AB,BD and AD of a parallelogram ABCD are yx+1=0, y+2x11=0 and y4x+7=0 respectively. Find the coordinates of A,B,C,D ... I have obtained some coordinates by performing simultaneous equation for D(3,5) A(2,1) B(4,3) To find C coordinates, according to the diagram I drew, since CB is perpendicular to AB so using m2=1/m1 I obtain the slope of CB inserting the coordinates of B, I obtained y+x=7. Doing the same thing for CD, which is perpendicular to AD, and inserting D coordinates, I obtained 4y+x=23. After picking both equations and solve, I obtain a wrong answer. Last edited by skipjack; August 31st, 2018 at 12:25 PM. 
August 31st, 2018, 12:01 PM  #2 
Global Moderator Joined: Dec 2006 Posts: 20,921 Thanks: 2203 
As ABCD is a parallelogram, you can't assume that CB is perpendicular to AB. As the diagonals AC and BD bisect each other, adding corresponding coordinates of B and D must give the same results as adding corresponding coordinates of A and C. Adding the coordinates of B and D gives (7, 8). Subtracting the coordinates of A from that gives C as the point (5, 7). 

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