Chords PQ and RS of a circle extended externally intersect at point T. Length of the segment PT, if lengths of QT, RT and ST are 9 cm, 18 cm and 6 cm respectively, is
Scientists believe that , about 4.6 billion years ago, the solar system was formed when a cloud of gas and dust in space was disturbed, maybe by the explosion of a nearby star called a supernova.