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9th Class Mathematics Surface Areas and Volumes Question Bank Surface Area and Volume

  • question_answer
    A cardboard sheet in the form of a circular sector of radius 20 cm and central angle\[\mathbf{10}{{\mathbf{8}}^{{}^\circ }}\] is folded to make a cone. What is the radius of the cone?

    A)  6 cm      

    B)  18 cm          

    C)  21 cm                         

    D)  4 cm

    Correct Answer: A

    Solution :

    (a): Here, area of sector, \[2\pi r\left( \frac{\theta }{{{360}^{{}^\circ }}} \right)=l\] Also, length of one \[l=2\pi r\] \[2\pi r=2\pi \times 20\times \frac{108}{360}\] \[\Rightarrow r=\frac{20\times 144}{360}=6\]cm             


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