Use the Formula $\frac { 1 } { f } = \frac { 1 } { s _ { 0 } } + \frac { 1 } { s _ { i } }$

Multiple Choice

Use the formula $\frac { 1 } { f } = \frac { 1 } { s _ { 0 } } + \frac { 1 } { s _ { i } }$ . Given $s _ { 0 } = 6$ cm and $s _ { i } = 12$ cm, find $f$ .

A) $f = 18 \mathrm {~cm}$
B) $f = 3 \mathrm {~cm}$
C) $f = 7 \mathrm {~cm}$
D) $f = 2 \mathrm {~cm}$
E) $f = 4 \mathrm {~cm}$