How do you prove this without using induction? How do you prove this without using induction
$$\frac{1}{n}+\frac{1}{n+1}+\frac{1}{n+2}+\cdots+\frac{1}{2n-1}=1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}+\cdots+\frac{1}{2n-1}$$

Let $A(n)=\frac 11 + \frac 12 + \frac 13 + \dots + \frac 1n$. We have 
$$
\begin{align}
A(2n)&=\frac 11 + \frac 12 + \frac 13 + \dots + \frac 1n + \dots + \frac 1{2n} \\
&=\left(\frac 11 + \frac 13 + \frac 15 + \dots + \frac 1{2n-1}\right)+\left(\frac 12 + \frac 14 + \frac 16 + \dots + \frac 1{2n}\right) \\
&=\left(\frac 11 + \frac 13 + \frac 15 + \dots + \frac 1{2n-1}\right)+\frac 12 A(n)
\end{align}
$$
However, we also have
$$
\begin{align}
A(2n)&=\frac 11 + \frac 12 + \frac 13 + \dots + \frac 1n + \dots + \frac 1{2n} \\
&=A(n)+\left(\frac 1{n+1} + \frac 1{n+2} +\dots + \frac 1{2n} + \dots + \frac 1{2n} \right)
\end{align}
$$
Thus 
$$
\begin{align}
A(n)+\left(\frac 1{n+1} + \frac 1{n+2} +\dots + \frac 1{2n} + \dots + \frac 1{2n} \right) &=\left(\frac 11 + \frac 13 + \frac 15 + \dots + \frac 1{2n-1}\right)+\frac 12 A(n) \\
\frac 1{n+1} + \frac 1{n+2} +\dots + \frac 1{2n} + \dots + \frac 1{2n} &=\left(\frac 11 + \frac 13 + \frac 15 + \dots + \frac 1{2n-1}\right)-\frac 12 A(n) \\
&= \frac 11 -\frac 12 + \frac 15-\frac 14 + \dots + \frac 1{2n-1}
\end{align}
$$
