You need a line of length √5, and at one end of it draw a perpendicular line of length 4. Join their ends to form the hypotenuse and applying the wisdom of Pythagoras this third line will be of length √80. 😁 😁 😁

But wait! Where do you find that first line of length √5? You must construct it also—as the hypotenuse of a ∆ having base=1 and whose height=2.

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 If you think my first answer sounds more complicated than you'd prefer, then just draw a right-angled triangle with base=4 and height=8 and Pythagoras says the hypotenuse will be √80. 😛

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