Question

Tìm x

a)1+3+5+7+...+(2x-1)=225

b)(21-3x).(2x-10).(6-2x)=0

Answer 1

a) 1 + 3 + 5 + 7 + ... + (2x - 1) = 225

  (2x - 1 + 1) . [(2x - 1 - 1) : 2 + 1] : 2 = 225

 2x.[(2x - 2) : 2 + 1] : 2 = 225

 x . (x - 1 + 1) = 225

x . x = 225

 => x = 15

Answer 2

b ) 1 + 3 + 5 + ... + ( 2x + 1 ) = 225

Dãy trên có : ( 2x + 1 - 1 ) : 2 + 1 = x + 1 ( số hạng )

( 2x + 1 + 1 ) . ( x + 1 ) : 2 = ( x + 1 ) . ( x + 1 ) = ( x + 1 )^2 = 225

Mà 225 = 15^2 

=> x + 1 = 15

    x = 15 - 1 

    x = 14

Answer 3

b) (21 - 3x).(2x - 10).(6 - 2x) = 0

TH1 : 21 - 3x = 0

     => 3x = 21 - 0

     => 3x = 21

     =>  x = 21 : 3 = 7

TH2: 2x - 10 = 0

       =>2x = 0 + 10

      => 2x = 10

     =>  x = 10 : 2 = 5

TH3: 6 - 2x = 0

      => 2x = 6 -0

      => 2x = 6

      => x = 6 : 2 = 3