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Draw a long horizontal line AB.
From A draw a line AC perpendicular to AB at A, of length 2 unit.  So C is at 2 units from A.  

Using compass measure 3 units.  With C as the center, draw an arc cutting AB at D.    Now AD will be √5 units.

There can be many ways.  The above is one way.
Using Pythagoras theorem , we can say that
     AD² = CD² - AC² = 3² - 2² = 5
     AD = √5  units

1 5 1
why should we take 2 units only?
wait i explain
i hope its clear
ya thnk u
thnx a lotttt