Question 52976
This idea drives a lot of students nuts.  When you have an expression, you don't have an equal sign in which to multiply both sides of the equation by a number and eliminate the denominators.
{{{x/2+x/3}}}
The best you can do is give the fractions a common denominator by multiplying the numerator and the denominator by a number that will give both fractions the same denominator and combine the numerators:
{{{3*x/(3*2)+2*x/(2*3)}}}
{{{3x/6+2x/6}}}
{{{5x/6}}}
Equations enable us to use the fact that we can multiply both sides of the equation by anything (except 0) and not change the value of the equation.
{{{x/2+x/3=1}}}
{{{6x/2+6x/3=6*1}}}
{{{3x+2x=6}}}
{{{5x=6}}}
{{{5x/5=6/5}}}
{{{x=6/5}}}
We can solve this same equation by simplifying the left side and then solving for x, but it's extra work with fractions and most of us don't like extra work especially when it involves fractions.  You'll recognize the first three lines from the example I gave you on simplified expressions:
{{{3*x/(3*2)+2*x/(2*3)=1}}}
{{{3x/6+2x/6=1}}}
{{{5x/6=1}}}
{{{(6/5)(5x/6)=1(6/5)}}}
{{{x=6/5}}}
I hope this helps.  In short, expressions don't have equal signs so we can't clear our fractions by multiplying both sides of the = sign by the LCD.