The goal is to get a common denominator and combine the fractions into one single fraction.
Notice that (a - c) is in the denominator of the first fraction, but not in the second. So we need to multiply and divide the second fraction by (a - c) . That is, multiply it by (a - c)/(a - c)
\(\frac{1}{(2a-b)(a-c)}+\frac{1}{(b-c)(b-2a)} \\~\\ =\frac{1}{(2a-b)(a-c)}+\frac{1(a-c)}{(b-c)(b-2a)(a-c)}\)
Now we need to multiply the first fraction by (b - c)/(b - c) .
\(=\frac{1(b-c)}{(b-c)(2a-b)(a-c)}+\frac{1(a-c)}{(b-c)(b-2a)(a-c)}\)
Now, notice that the term (2a - b) is -1 multiplied by (b - 2a) .
So let's multiply the second fraction by -1/-1 .
\(=\frac{b-c}{(b-c)(2a-b)(a-c)}+\frac{(-1)(a-c)}{(b-c)(-1)(b-2a)(a-c)} \\~\\ =\frac{b-c}{(b-c)(2a-b)(a-c)}+\frac{-a+c}{(b-c)(-b+2a)(a-c)} \\~\\ =\frac{b-c}{(b-c)(2a-b)(a-c)}+\frac{-a+c}{(b-c)(2a-b)(a-c)}\) And distribute it.
Now the denominators are the same and we can add the fractions together.
\(=\frac{b-c+-a+c}{(b-c)(2a-b)(a-c)} \\~\\ =\frac{b-a}{(b-c)(2a-b)(a-c)}\) -c and +c cancel