Bài 1: Tính

a) $\dfrac{5}{6}$ + $\dfrac{1}{6}$ : $\dfrac{2}{3}$ = $\dfrac{5}{6}$ + $\dfrac{1}{6}$ x $\dfrac{3}{2}$ = $\dfrac{5}{6}$ + $\dfrac{3}{12}$ = $\dfrac{10}{12}$ + $\dfrac{3}{12}$ = $\dfrac{13}{12}$

b) $\dfrac{7}{15}$ x $\dfrac{20}{21}$ x $\dfrac{3}{11}$ = $\dfrac{420}{3465}$ = $\dfrac{28}{231}$

Bài 2: Rút gọn phân số

a) $\dfrac{54}{108}$ = $\dfrac{54 : 54}{108 : 54}$ = $\dfrac{1}{2}$

b) $\dfrac{303}{404}$ = $\dfrac{303 : 101}{404 : 101}$ = $\dfrac{3}{4}$

c) $\dfrac{123123}{246246}$ = $\dfrac{123123 : 1001}{246246 : 1001}$ = $\dfrac{123}{246}$ = $\dfrac{123 : 3}{246 : 3}$ = $\dfrac{41}{82}$

Bài 3: So sánh: $\dfrac{1995 * 1995}{1991 * 1999}$ với 1

$\dfrac{1995 * 1995}{1991 * 1999}$ 

= $\dfrac{(1991 + 4) * 1995}{1991 * (1995 + 4)}$

= $\dfrac{1995 * 1995 + 4 * 1995}{1991 * 1995 + 1991 * 4}$

= $\dfrac{1995 * 1995 + 7980}{1991 * 1995 + 7964}$

Vì 7980 > 7964 nên phân số $\dfrac{1995 * 1995}{1991 * 1999}$ lớn hơn 1

$\text{Bài 1:}$

a) $\frac{5}{6}$ + $\frac{1}{6}$ : $\frac{2}{3}$ 

$=$$\frac{5}{6}$ + $\frac{1}{6}$ × $\frac{3}{2}$ 

$=$ $\frac{10}{12}$ + $\frac{3}{12}$ 

$=$ $\frac{13}{12}$ 

b) $\frac{7}{15}$ × $\frac{20}{21}$ × $\frac{3}{8}$ 

$=$ $\frac{7×20×3}{15×21×8}$ 

$=$ $\frac{7×(4×5)×3}{(3×5)×(7×3)×(2×4)}$ 

$=$ $\frac{1}{6}$ 

$\text{Bài 2:}$

a) $\frac{54}{108}$ $=$ $\frac{54}{2×54}$ $=$ $\frac{1}{2}$ 

b) $\frac{303}{404}$ $=$ $\frac{101×3}{101×4}$ $=$ $\frac{3}{4}$ 

c) $\frac{123123}{246246}$ $=$ $\frac{123123}{123123×2}$ $=$ $\frac{1}{2}$ 

$\text{Bài 3:}$ $\frac{1995×1995}{1991×1999}$ và 1

$\frac{1995×1995}{1991×1999}$ $=$ $\frac{(1991+4)×1995}{(1995+4)×1991}$ $=$ $\frac{1991×1995+1995×4}{1991×1995+1991×4}$ 

Vì 1991 < 1995 nên 1991 × 4 < 1995 ×4 

⇒$1991×4+1991×1995 < 1995×4+1991×1995$

⇒ $\frac{1991×1995+1995×4}{1991×1995+1991×4}$ $<1$

Vậy $\frac{1995×1995}{1991×1999}$ $<1$

#keishathomas