Calculation: y7l47r - 1

1

/ d

-

1

/ f

=
-

1

/ F

1

/ d

-

1

/ f

= -

1

/ F

$$\frac{1}{d}-\frac{1}{f}$$ = $$-\frac{1}{F}$$

1

/ d

= -

1

/ F

+

1

/ f

$$\frac{1}{d}$$ = $$-\frac{1}{F}+\frac{1}{f}$$

d =

1

/ (-

1

/ F

+

1

/ f

)

$$d$$ = $$\frac{1}{-\frac{1}{F}+\frac{1}{f}}$$