sumbody explain me this ... again .. in an easier manner 
manbir...we've done this question in the class...don't you remember??
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and its not from add. maths shreycool....its from our syllabus..as in igcse maths
manbir....simply draw a line BD....
then using a D measure a 90 degrees angle at B for BD....
draw a line of the same length as BD along the 90 degree anglee...mark this point F
then draw a curve from D to F...dts the locus....
isn't that rite guys???