Two lines are said to be mutually perpendicular when the angle between them is 90 deg. or the tangent of the angle between them is infinity . (since tan 90 = infinity)

 slope of x-axis is m1 = 0

slope of y-axis is m2 = infinity

Now  if angle between x and y axis is 0, then tan0 =( m1+m2)/(1-m1.m2)

                                                                                         =( 0+inf.)/1-0.inf)

                                                                                         = inf./1

                                                                                         =inf.

therefore x and y axis are perpendicular to each other .

 Hello brother 

check out the same question posted in this link.....and have a look on my solution....you will get clear 

http://www.goiit.com/posts/list/analytical-geometry-hello-friends-one-interesting-question-for-990302.htm