Question

Tìm x :

      x + ( x + 1 ) + ( x + 2 ) + ( x +3 ) + ......+ ( x + 30 ) = 1240

  Ai giúp mình , mình tích cho .

Answer 1

) x+(x+1)+(x+2)+…+(x+30)=1240

=>x+x+1+x+2+x+3+…+x+30=1240

=>x+x+x+…+x+1+2+3+…+30=1240

Từ 1->30 có: (30-1):1+1=30(số)

=>31.x+(30+1).30:2=1240

=>31.x+31.15=1240

=>31.x+465=1240

=>31.x=1240-465

=>31x=775

=>x=775:31

=>x=25

Answer 2

x + ( x + 1 ) + ( x + 2 ) + ( x + 3 ) + ... + ( x + 30 ) = 1240

( x + x + x + x + ... + x ) + ( 0 + 1 + 2 + 3 + ... + 30 ) = 1240

Đặt A = 0 + 1 + 2 + 3 + ... + 30

Dãy A có số số hạng là :

     ( 30 - 0 ) : 1 + 1 = 31 ( số )

=> B = x + x + x + x + ... + x có 31 số x

Ta có :

( x + x + x + x + ... + x ) + ( 0 + 1 + 2 + 3 + ... + 30 ) = 1240

31x                             + ( 0 + 30 ) x 31 : 2               = 1240

31x                             + 465                                    = 1240

31x                                                                          = 775

         x                                                                     = 25

Answer 3

x + ( x + 1 ) + ( x + 2 ) + ( x +3 ) + ......+ ( x + 30 ) = 1240

(x + x + x + x +....+ x) + (1 + 2 + 3 + .... + 30) = 1240

31x + 930 = 1240

31x = 1240 - 930

31x = 310

x = 310 : 31

x = 10