Question

explainn how to construct a 45 dgree angle using ruler and compass only

Answer 1

first bisect a line using a compass with span greater than half length of the line
and draw the bisector using a ruler
now draw the bisector of the 90 degree angle
from the point of intersection draw an arc with the compass - the arc must pass through the two lines
then place the point of compass at one point of intersection and draw another arc between 2 lines
repeat on other point of intersection and draw arc which intersects the arc just drawn
finally draw a line from intersection of line and its bisector through the intersectiopn of two arcs

Answer 2

What jimmy said:
http://www.youtube.com/watch?v=7lCa9zqti4o

Answer 3

this of course presupposes that you are drawing it on a perfectly flat surface ;)