Prerequisites:
Median (definition)
Midpoint Theorem (proof)
SAS congruence (proof)
Angle on a straight line (proof)
Corresponding angles property (proof)
Proof:
Let \triangle ABC be a right triangle, right angled at B. Let BD be a median drawn from B to meet AC at D.
We need to show that BD = \dfrac{1}{2} AC. For this, let us join DE, where E is the midpoint of AB.
Since, D and E are the midpoints of AC and AB respectively (see definition of median), hence by Midpoint Theorem,
\begin{align}\qquad\quad &DE \parallel BC\\ \Rightarrow\quad\;\; &\angle AED = \angle ABC = 90^o\quad\qquad\qquad\!\! && \text{(corresponding angles)} &&& \cdots\text{(1)}\\ \Rightarrow\quad\;\; & \angle BED = 180^o -\angle AED = 90^o && \text{(angle on a straight line)} &&& \cdots\text{(2)}\end{align}
Now, consider \triangle AED and \triangle BED:
\begin{align}\qquad\quad\;\; & AE = BE\qquad\qquad\qquad\qquad\qquad && \text{(by construction)}\\ & DE = DE && \text{(common)}\\ & \angle AED = \angle BED = 90^o && \text{(from $(1)$ and $(2)$)}\\ \therefore\quad\;\;\;\;\; & \triangle AED\cong\triangle BED && \text{(by SAS congruence)}\\ \\[12pt] \Rightarrow\;\quad\quad & AD = BD && \text{(CPCTC)}\qquad\qquad\qquad\qquad\qquad\cdots\text{(3)}\\ \text{But, }\;\quad & AD = AC/2 && \text{(by definition of median)}\\ \text{Hence, }\; & BD = AC/2 && \text{(from $(3)$)}\end{align}
Q.E.D.
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