If :

(20x−10).8.(15x+2)=0

=> 20.x - 10  = 0 or 15.x + 2 = 0 

* 20 .x -10 = 0                                * 15 . x + 2 = 0

   20 . x      =  0 + 10                           15 .x        = 0 - 2  

   20 . x       = 10                                  15 .x        = - 2 

          x       = 10 : 20                                 x         = - 2 : 15

           x      =  12

                                        x         = −215

So : x =  12

   ; x =  −215

Chúng ta có:

 1755. 2 = 3510 6019. 2 = 12038

1755 + 3510 = 4265 6019 + 12038 = 18057                                                      

 Câu trả lời là :

8426. 2 = 16852

8426 + 16852 = 25278 

=>? = 25278

If x = 2 , we have x2 + 2x = 8 ( scrap because 8 isn't a prime number)

If x = 3 , we have x2 + 2x = 17 ( satisfy )

If x > 3 and x is a prime number then x =3k + 1 or 3k + 2 ( k is a natural number)

+) x = 3k +1

If x = 3k +1 then x2 divide to 3 has the remainder 1

and 2x divide to 3 has the remainder 2

=> 2x + x2 divided to 3 

But 2x + x2 > 3 

So 2x + x2 isn't a prime number

Do the similar to x = 3k+2

The answer is x = 3