Cách làm cho bạn:
a) $\frac{x + 1}{3}$ + $\frac{2x - 1}{3}$ = $\frac{x + 1 + 2x - 1}{3}$ = $\frac{3x}{3}$ = x;
b) $\frac{5x – 2y}{x^{2} - y^{2}}$ + $\frac{y – 4x}{x^{2} - y^{2}}$ = $\frac{5x – 2y + y – 4x}{(x – y)(x + y)}$ = $\frac{x - y}{(x – y)(x + y)}$ = $\frac{1}{x + y}$;
c) $\frac{x - 1}{12x}$ + $\frac{2x + 7}{12x}$ + $\frac{6 – 3x}{12x}$ = $\frac{x – 1 + 2x + 7 + 6 – 3x}{12x}$ = $\frac{12}{12x}$ = $\frac{1}{x}$.
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