a. => 16-2.(15-x)=10

=> 2.(15-x)=16-10

=> 2.(15-x)=6

=> 15-x=3

=> x=15-3

=> x=12

b. => 49-7.(13-x)=14

=> 7.(13-x)=35

=> 13-x=5

=> x=8

c. => 5x-25=10

=> 5x=10+25

=> 5x=35

=> x=35:5

=> x=7

d. =>14x-98=42

=> 14x=42+98

=> 14x=140

=> x=140:14

=> x=10.

A=5.(2.3+4.9.5+6.9.7+10.21.8)/7.(2.3+4.9.5+6.9.7+10.21.8)

A=5/7

A = 5/7

A = \(\dfrac{2.3.5+4.9.35+6.9.35+10.21.40}{2.3.7+4.9.35+6.9.49+10.21.56}\)

A = \(\dfrac{2.3.5+\left(2.3.5\right).2.3.5+\left(2.3.5\right).3.3.7+\left(2.3.5\right).5.7.8}{2.3.7+\left(2.3.7\right).2.3.5+\left(2.3.7\right).3.3.7+\left(2.3.7\right).5.7.8}\)

A = \(\dfrac{2.3.5.\left(1+2.3.5+3.3.7+5.7.8\right)}{2.3.7\left(1+2.3.5+3.3.7+5.7.8\right)}\)

A = \(\dfrac{2.3.5}{2.3.7}\)

A = \(\dfrac{5}{7}\)

Ta có: \(A=\dfrac{2\cdot3\cdot5+4\cdot9\cdot35+6\cdot9\cdot35+10\cdot21\cdot40}{2\cdot3\cdot7+4\cdot9\cdot35+6\cdot9\cdot49+10\cdot21\cdot56}\)

\(=\dfrac{2\cdot3\cdot5+2\cdot3\cdot5\cdot6\cdot7+2\cdot3\cdot5\cdot3\cdot3\cdot7+2\cdot3\cdot5\cdot5\cdot7\cdot8}{2\cdot3\cdot7+2\cdot3\cdot7\cdot2\cdot3\cdot5+2\cdot3\cdot7\cdot3\cdot3\cdot7+2\cdot3\cdot7\cdot5\cdot7\cdot8}\)

\(=\dfrac{2\cdot3\cdot5\cdot\left(1+2\cdot3\cdot7+3\cdot3\cdot7+5\cdot7\cdot8\right)}{2\cdot3\cdot7\cdot\left(1+2\cdot3\cdot5+3\cdot3\cdot7+5\cdot7\cdot8\right)}\)

\(=\dfrac{5}{7}\cdot\dfrac{386}{374}=\dfrac{965}{1309}\)

=95136

Ta có: 5.7.9.11 ⋮7 và 2.3.7 ⋮7 nên (5.7.9.11 – 2.3.7) ⋮7

   Vì 5.9.11 > 2.3 ⇒ 5.9.11 – 2.3 > 1 nên (5.7.9.11 – 2.3.7) > 7

   Vậy 5.7.9.11 – 2.3.7 là hợp số

Ư(A)={1;2;3;4;7;6;12;14;21;28;42;84}