10th Class Mathematics Triangles Question Bank Triangles

  • question_answer
    ABC is right triangle right angled at C. If p is the length of the perpendicular from C to AB and a, b, c have the usual meaning, then \[\frac{1}{{{a}^{2}}}+\frac{1}{{{b}^{2}}}=\]

    A)  \[1/{{p}^{2}}\]                   

    B)  \[2/{{p}^{2}}\]       

    C)  \[{{p}^{2}}\]                      

    D)         \[2{{p}^{2}}\]        

    Correct Answer: A

    Solution :

    Area of \[\Delta ABC=\frac{1}{2}\times c\times p=\frac{1}{2}\times b\times a\] \[\Rightarrow \]            \[cp=ba\,\,\Rightarrow \,\,\,\frac{1}{{{p}^{2}}}=\frac{{{c}^{2}}}{{{a}^{2}}{{b}^{2}}}\] But, \[{{c}^{2}}={{a}^{2}}+{{b}^{2}}\][By Pythagoras theorem] \[\therefore \]  \[\frac{1}{{{p}^{2}}}=\frac{{{a}^{2}}+{{b}^{2}}}{{{a}^{2}}{{b}^{2}}}\,\,\Rightarrow \,\,\frac{1}{{{a}^{2}}}+\frac{1}{{{b}^{2}}}=\frac{1}{{{p}^{2}}}\]


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