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# Challenge problems: perimeter & area

Three example problems involving perimeter and area. Created by Sal Khan.

## Want to join the conversation?

• Can't you just multiply 30x5?
• if you multiply it by 30x5 what you get is the perimeter of all things which include the bases which look like a trapezoid since these sides of 5 sides structure isnt part of the whole perimeter we need to subtract it which the perimeter of the 5 side which is 50 and we just need to get all sides including the 5 side structure-the 5 side structure perimeter which is 150-50=100
• At , how does the fact that all the angles are 90º tell us that those two sides have equal lengths?
• You can ask it in Khan Academy Community. I think that Sal made a mistake. (I'm not a Khan Academy Staff)
• For the second example at ,couldn't you just use the area for calculating the area of a trapezoid?
(parallel side 1 +parallel side 2,/2 ,times height)
• Pardon me if I'm overseeing something, but around , Sal says that the lengths of the two circled sides are both 2. How does he know that? Couldn't the white side be a bit longer, and then the purple side would be shifted down and the circled side on the left would be shorter?
• instead of doing that much calculation for the trapezoid wouldn't it just be easier to do (a+b)h/2
• Actually, that is another way of finding a trapezoid's area. The multiplying by one half is the same as dividing by two.
• Is there a difference between a trapezium and a trapezoid
• Nope not really they mean the same thing just in different places
(1 vote)
• If all you know is the perimeter of the trapezoid(70mm)how do you find the area?
• You can't find the area of a non-regular polygon based on just the perimeter. E.g. Given a rectangle with perimeter 16. If it was a 4x4 square, the area would be 16. If it was a 7x1 rectangle, it would be 7. 16 and 7 are very different areas, but both shapes have the same perimeter.
• I cant get anything right on khan academy. EVER! I swear im doing the right thing!
• You cant be doing it right if you have them wrong, are you double checking your work and just get math wrong?
(1 vote)
• At , to find the perimeter wouldn't Sal have to include the 9 for the perimeter of the square?