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if pab is asecant to circle …

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if pab is asecant to circle intersecting the circle at a and b abd pt is atangent segment then prove that pa.pb=pt^2
  • 1 answers

Sia ? 6 years, 5 months ago

 

 

  1. PA .PB = (PN - AN)(PN + BN)
    = (PN - AN) (PN + AN) {tex}\left[ \begin{array} { c } { \because O N \perp A B } \\ { \therefore N \text { is the mid-point of } A B } \\ { \Rightarrow A N = B N } \end{array} \right]{/tex}
    = PN2 - AN2
  2. Applying Pythagoras theorem in right triangle PNO, we obtain
    OP2 = ON2 + PN2
    {tex}\Rightarrow{/tex}PN2 = OP2 - ON2
    {tex}\therefore{/tex} PN2 - AN2 = (OP2 - ON2) - AN2
    = OP2 - (ON2 + AN2)
    = Op2 _ OA [Using Pythagoras theorem in {tex}\Delta O N A{/tex}]
    = OP2 - OT2 {tex}[ \because O A = O T = \text { radius } ]{/tex}
  3. From (i) and (ii), we obtain
    PA.PB = PN2 - AN2 and PN2 - AN2 = OP2 - OT2
    {tex}\Rightarrow{/tex} PA .PB = OP2 - OT2
    Applying Pythagoras theorem in {tex}\triangle O T P{/tex}, we obtain
    OP2 = OT2 + PT2
    {tex}\Rightarrow{/tex} OP2 - OT2 = PT2
    Thus, we obtain
    PA.PB = OP2 - OT2
    and OP2 - OT2 = PT2
    Hence, PA.PB = PT2
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