# 1/s+1/i=1/f, need i = ...

#### BearishSun

##### New member
Hello,

how do I get equation "1/s+1/i=1/f" in the form i = ... ?

The solution is i = f*s/(s-f), but I'd like to know the steps for getting the solution.

#### galactus

##### Super Moderator
Staff member
Find your LCD. It is $$\displaystyle sif$$, isn't it?.

$$\displaystyle \frac{if}{if}\cdot\frac{1}{s}+\frac{sf}{sf}\cdot\frac{1}{i}=\frac{si}{si}\cdot\frac{1}{f}$$

Now that the denominators are the same, concentrate on the numerators.

$$\displaystyle if+sf=si$$

Now, solve for i.

#### BearishSun

##### New member
So simple...still, I doubt it would occur to me

Thanks again

#### Subhotosh Khan

##### Super Moderator
Staff member
Too many ifs in the problem .... :mrgreen: