Question 79598
Simplify:
{{{R[t] = 1/r[1] + 1/r[2]}}} Add the fractions as you would add any other fractions with unlike denominators.
Find a commom denominator.  Here, it would be {{{(r[1])*(r[2])}}}
Then, here, multiply the first fraction by {{{r[2]/r[2]}}} and the second fraction by {{{r[1]/r[1]}}}
{{{R[t] = r[2]/r[1]r[2] + r[1]/r[1]r[2]}}} Now you can add the fractions because they have a common denominator.
{{{R[t] = (r[1]+r[2])/r[1]r[2]}}}