Cách làm cho bạn:
a) $\frac{1}{x}$ - $\frac{1}{x + 1}$ = $\frac{x + 1}{x(x + 1)}$ - $\frac{x}{x(x + 1)}$ = $\frac{x + 1 - x}{x(x + 1)}$ = $\frac{1}{x(x + 1)}$;
b) $\frac{1}{xy - x^{2}}$ - $\frac{1}{y^{2} - xy}$ = $\frac{1}{x(y – x)}$ - $\frac{1}{y(y – x)}$ = $\frac{y}{xy(y – x)}$ - $\frac{x}{xy(y – x)}$ = $\frac{y - x}{xy(y – x)}$ = $\frac{1}{xy}$.
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