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The square abcd has side length 10cm. E is the midpoint of BC. Work out the length of DE. Answer to 1 decimal place.

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IwAsSoMeGuY
Think of it as a triangle. E is the midpoint of BC, so EC is 5cm. Then CD is 10cm. Pythagoras it, so 5^2 + 10^2 = DE^2. Rt(125) = 11.2 to 1 decimal place. That's the answer
AL2006

-- DEC is a right triangle.

-- DC is one side of the square  =  10 cm
-- EC is half the side of the square = 5 cm
-- DE is the hypotenuse = square root of (5² + 10²)  =  √(125 cm²) .

-- Truncated to one decimal place, that's  11.1 cm. 

-- Rounded to one decimal place, it's 11.2 cm.