February 18th, 2015, 06:28 AM  #1 
Newbie Joined: Feb 2015 From: Dhaka Posts: 4 Thanks: 0  using pythagoras theorem (3)
In parallelogram ABCD, the diagonals AC is at right angles to AB.If AB=12 & AC=13.Find the area of parallelogram.(use Pythagoras theorem)?i am also confused with the diagram.
Last edited by greg1313; February 18th, 2015 at 09:37 AM. 
February 18th, 2015, 06:33 AM  #2 
Math Team Joined: Jan 2015 From: Alabama Posts: 2,422 Thanks: 614 
The two diagonals divide the parallelogram into four right triangles. The area of a right triangle is 1/2 times the product of the lengths of the legs.

February 18th, 2015, 07:23 AM  #3 
Senior Member Joined: Apr 2014 From: Glasgow Posts: 1,965 Thanks: 640 Math Focus: Physics, mathematical modelling, numerical and computational solutions 
The question is slightly wrong. It should be "In parallelogram ABCD, the diagonal AC is at rightangles to the diagonal BD. If AB=12 and AC=13, find the area of the parallelogram. You will need to use Pythagoras' theorem." Country Boy's suggestion is good. If you still find it difficult, here's some additional advice, especially with drawing the diagram and labelling it properly (it might seem very simple, but it's important so I'm going to put it anyway!). Follow it through stepbystep. 1. Draw a parallelogram. If you can, draw it so that the base and the top is flat and sides are slanted. It's just easier to visualise and work with. It should look something like this: 2. Use a ruler! 3. Label the corner points A, B, C and D. In maths, shapes are often named by listing the points that join up all the sides together, so your shape now has a name 'ABCD'. 4. The question mentioned a line 'AC'. 'AC' is a line that joins the points A and C together, so get your ruler out and draw the line. This line is called a 'diagonal' because it goes across the whole shape at an angle, rather than along one of the sides. 5. The other diagonal is BD. Go and draw the line 'BD' in. 6. AC is at rightangle BD, so look at the point where the lines join together and put a little square there to denote a rightangle. 7. The question states that AB = 12 and AC =13, so put these numbers by the lines. Okay... so far so good... now ask yourself the following: 1. Can you see any rightangled triangles? 2. What are the lengths of the sides of the triangles? 3. What sides of the triangles are unknown? 4. Can they be calculated? 5. What is the area of the rightangled triangle? 6. How can you get the area of ABCD using the areas of the rightangled triangles? Post again if you get stuck Last edited by Benit13; February 18th, 2015 at 07:32 AM. 
February 18th, 2015, 10:57 AM  #4 
Global Moderator Joined: Dec 2006 Posts: 16,947 Thanks: 1255  

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