Надо привести дроби к общему знаменателю.
а>b, если (a-b)>0
1,\; \; \frac{645}{646}<1\; \; \to \; \; \frac{72}{71}>\frac{645}{646}" alt="0,99-\frac{199}{201}=\frac{99}{100}-\frac{199}{201}=\frac{99\cdot 201-199\cdot 100}{100\cdot 201}=-\frac{1}{20100}<0\; \to \\\\0,99<\frac{199}{201}\\\\\\\frac{72}{71}>1,\; \; \frac{645}{646}<1\; \; \to \; \; \frac{72}{71}>\frac{645}{646}" align="absmiddle" class="latex-formula">