• # question_answer    The length of an elastic string is x when the tension is 5N. Its length is y when the tension is 7N. What will be its length, when the tension is 9N?                                 A)  $2y+x$                      B)  $2y-x$C)  $7x-5y$                      D)  $7x+5y$

We have, $F=kx$ where, F, x and k are force, length and constant respectively.             $\therefore$      $5=kx$                                   ?.(1)             and      $7=ky$                                   ?.(2) Multiplying eq. (2) by 2 $14=2ky$ Subtracting eq (1) from (3), $14-5=2ky-kx$ or $9=k(2y-x)$ Hence, required length $=2y-x$