x . 5 + 20 . x + 40 : 2 = 70

x . (5 + 20) + 20 = 70

x . 25 + 20 = 70

x . 25 = 70 - 20

x . 25 = 50

x = 50 : 25

x = 2

Very good

Tôi không biết nha 

Ta has: abcabc = abc000 + abc 

                      = Abc x 1000 + abc 

                      = Abc. (1000 +1)

                      = Abc. 1001

                      = Abc. 7. 11. 13

The abcabc of the abc with 7; 11; 13 => abcabc divide by 7; 11 and 13

Thank you . You should post another question

a, 1 + 2 + 3 + 4  5 + 6 + 7 + 8 + 9

= (1 + 9) + (2 + 8) + (3 + 7) + (4 + 6) + 5

= 10 + 10 + 10 + 10 + 5

= 40 + 5

=45

b, 10 + 20 + 30 + 40 + 50 + 60 + 70 + 80 + 90

= (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9) x 10

= 45 x 10

= 450

We have:

A = -2 (3x - 2y) - 5 (2y - 3x) = -2 (3x - 2y) + 5 (3x - 2y)

    = 3 (3x - 2y) = 9x - 6y

B = (4x² - 3y) . 2y - (3x² - 4y) . 3y = 8x² . y - 6y² 9x² . y + 12y²

    = 6y² - x² . y

We have : n = a² + b² (a ; b e z) then:

2n = 2a² + 2b² = a² - 2ab + b² + a² + 2ab + b² = (a - b)² + (a + b)²

10 + 30 + 70 : 90

= 10 + 30 + \(\dfrac{7}{9}\)

= \(\dfrac{367}{9}\) 

2 + 3 + 4 + 5 + 100 + 6

= (2 + 3 + 5) + (4 + 6) + 100

= 10 + 10 + 100

= 20 + 100

=120

10 + 20 : 5 + 6 + 90 + 100

= 10 + 4 + 6 + 90 + 100

= (10 + 90) + (4 + 6) + 100

= 100 + 10 + 100

= 200 + 10

=210

The number of 1-digit numbers is : (9 - 1) + 1 = 9 

The number of 2-digit numbers is : (99 - 10) + 1 = 90

The number of 3-digit numbers is : (321 - 100) + 1 = 222

The number of digits she wrote is : 

9 x 1 + 90 x 2 + 222 x 3 = 855

Answer : 855

The satisfied numbers are divisible by 5 and 17, so they're divisible by 85. The number of positive satisfied numbers is :

(935 - 170) : 85 + 1 = 10 

The answer is :

 10 x 2 = 20 (integers)

The general forms of the satisfied palindromes are 

The number of 2-digit palindromes is : (99 - 11) : 11 + 1 = 9

There are 9 choices to choose c and 10 choices to choose d, so the number of 3-digit palindromes is : 9 x 10 = 90

The only 4-digit palindrome is 1001

So, the answer is : 9 + 90 + 1 = 100 (palindromes)

The answer is:

S(48) = 48 + 24 + 16 + 12 + 8 + 6 + 4 + 3 + 2 + 1 = 124

Let x, 7 - x be the  97^th and the 98^th term of the sequence respectively, then 14 - x ; 21 - x ; 35 - 2x are the 100^th ,101^st ,102^nd terms of the sequence respectively

We have:

35 - 2x = 69 <=> 2x = -34 <=> x = -17 <=> 14 - x = 31

So the answer is 31

The first reflection changes  P(-3; -2) to (2; 3)

The answer is: 

(2 + 4; 3 - 1) = (6; 2)

x + 10 = 20 + 30

x + 10 = 50

x = 50 - 10

x = 40

3 x 10 + 3 x 20

=3 x ( 10 + 20 )

=3 x 30

=90

The greatest divisor of 6 is 6