what, you can't plug in the numbers?
1/f = 1/(7/2) - 1/(15/2)
1/f = 2/7 - 2/15
1/f = (30-14)/105
1/f = 16/105
f = 105/16
given the mirror formula 1/f=1/v-1/u,f is the focal length of the lens u and v are object and image distance respectively .find f when u and v are 7.5cm and 3.5cm respectively
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