Consider triangle AOB ln, the x-y plane ,where A(1,0,0),,B(0,2,0), and O(0,0,0).The new position of O, when triangle is rotated about side AB by 90o can be
Equation of line AB is
Now
From (i) and (ii), we get
From (iii), putting
Putting this value of x1, and y1 in (i),
we get