Bài 1 :
`a) 37 - ( x + 4 ) = 20`
`⇒ x + 4 = 17`
`⇒ x = 13`
Vậy `x = 13`
`b) ( 6x - 9 ) - 12 = 8`
`⇒ 6x - 9 = 20`
`⇒ 6x = 29`
`⇒ x =` $\frac{29}{6}$
Vậy ` x =` $\frac{29}{6}$
c) $\frac{1}{3}$.x - $\frac{3}{5}$ = $\frac{2}{6}$
⇒ $\frac{1}{3}$.x = $\frac{14}{15}$
⇒ x = $\frac{14}{5}$
Vậy x = $\frac{14}{5}$
d) 2. ( x - 1 ) + 5 = x + 6
⇒ 2x - 2 = x + 1
⇒ 2x = x + 3 ( cùng trừ đi mỗi vễ x )
⇒ x = 3
Vậy x = 3
Bài 2:
a) 79. 31 + 21. 31
= 31. ( 79 + 21 )
= 31. 100
= 3100
b) 33. 27 - 33. 24
= 33. ( 27 - 24 )
= 33. 3
= 99
c) 49. 6 + 98. 47
= 49. 6 + 49. 2. 47
= 49. ( 6 + 2. 47 )
= 49. ( 6 + 94 )
= 49. 100
= 4900
d) $\frac{3}{7}$ . $\frac{1}{4}$ + $\frac{3}{7}$.$\frac{5}{4}$ + $\frac{2}{7}$. $\frac{3}{4}$
= $\frac{3}{28}$ + $\frac{15}{28}$ + $\frac{6}{28}$
= $\frac{3 + 15 + 6}{28}$
= $\frac{24}{28}$
= $\frac{6}{7}$
e) $\frac{1}{2}$ + $\frac{1}{6}$ + $\frac{1}{12}$ + $\frac{1}{20}$ + $\frac{1}{30}$ + $\frac{1}{42}$ + $\frac{1}{56}$ + $\frac{1}{72}$
= $\frac{1}{1. 2}$ + $\frac{1}{2. 3}$ + $\frac{1}{3. 4}$ + $\frac{1}{4. 5}$ + $\frac{1}{5. 6}$ + $\frac{1}{6. 7}$ + $\frac{1}{7. 8}$ + $\frac{1}{8. 9}$\
= 1 - $\frac{1}{2}$ + $\frac{1}{2}$ - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{4}$ + ... + $\frac{1}{8}$ - $\frac{1}{9}$
= 1 - $\frac{1}{9}$
= $\frac{8}{9}$