a) x.31+(1+2+3+4+.....+30)=1240

x.31+465=1240

x.31=1240-465

x.31=775

x=775:31

x=25

b) (1+x).x:2=210

(1+x).x=210.2

(1+x).x=420

Vì 1+x và x là 2 số nhiên liên tiếp và 420=20.21

=> x=20

a) 31x + (1+2+...+30) = 1240

   31x + 31.15 = 1240

  31x =1240 - 465

   31x = 775

   x= 25

b) (1+x) .x:2 = 210

  (x+1)x =210.2 = 21.20

 x =20

a) x + [x + 1] + [x + 2] + ... + [x + 30] = 1240

=> 31 . x + (1 + 2 + 3 + 4 +...+ 29 + 30) = 1240 
31 . x + 31.15 = 1240 
31 . x = 1240 - 31.15 
31 . x = 775 
x = 775 : 31 
x = 25 

b) 1 + 2 + 3 + ... + x = 210

=< x . (x + 1) = 201 . 2

=> x . (x + 1) = 420

Vì 420 = 20 . 21 nên x = 20

bai nay a de 

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

31x+465=1240

31x=1240-465

31x=775

x=775/31

x=25

b)1+2+3+...+x=210

Đặt A=1+2+3+...+x

A có: (x-1)+1=x(số hạng)

A=(x+1)*x/2=210

(x+1)*x=210*2

(x+1)*x=420

=>x=20

a) \(x+\left(x+1\right)+......+\left(x+30\right)=1240\\ \Rightarrow31x+465=1240\\ \Rightarrow31x=775\\ \Rightarrow x=25\)

b) \(1+2+3+.........+x=210\\ \Rightarrow\frac{\left(x+1\right)x}{2}=210\\ \Rightarrow\left(x+1\right)x=420\\ \Rightarrow x=20\)

a)

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

x + x + 1 + x + 2 + ... + x + 30 = 1240

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

(x . [30 - 1 + 1 + 1]) + ([30 + 1] . [30 - 1 + 1] : 2) = 1240

31x + 465 = 1240

           31x = 1240 - 465

           31x = 775

                x = 775 : 31

                x = 25

b) 1 + 2 + 3 + ... + x = 210

(x + 1) . (x - 1 + 1) : 2 = 210

     (x + 1) . (x - 1 + 1) = 210 . 2

     (x + 1) . (x - 1 + 1) = 420

                   (x + 1) . x = 420

Mà 20 . 21 = 420 => x = 20

a)

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

x + x + 1 + x + 2 + ... + x + 30 = 1240

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

(x . [30 - 1 + 1 + 1]) + ([30 + 1] . [30 - 1 + 1] : 2) = 1240

31x + 465 = 1240

           31x = 1240 - 465

           31x = 775

                x = 775 : 31

                x = 25

b) 1 + 2 + 3 + ... + x = 210

(x + 1) . (x - 1 + 1) : 2 = 210

     (x + 1) . (x - 1 + 1) = 210 . 2

     (x + 1) . (x - 1 + 1) = 420

                   (x + 1) . x = 420

Mà 20 . 21 = 420 => x = 20

a) 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

b) 1+2+3+…+x=210

=>x.(x+1):2=210

=>x.(x+1)=420

=>x.(x+1)=20.21=20.(20+1)

=>x=20

Bài 1:

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

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

31x + 465 =1240

31x = 1240 - 465

31x = 775

x = 775 : 31

x = 25

b, 1+2+3+...+x=210

\(\frac{x.\left(x+1\right)}{2}=210\)

x(x+1)=210.2

x(x+1)=420

x(x+1)=20.21

=>x=20

BAi 1:

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

x + x + x +...+ x + 1 + 2 + 3 + 4 +..........+ 30 =1240

day so 1 + 2 + 3 + ... + 30 co so so hang la:

(30-1):1+1=30

a, x + ( x+ 1) + ( x + 2 )+...+( x +  30 )= 1240

1240=31x + (1+2+...+30)

 1240=31x +{[(30-1)+1]:2} .(30+1)

1240=31x +15.31

1240=31x+465

31x=1240-465

31x=775

x=775 : 31

x=25

vay x=25

k mình nha

a) x=25

b) SSH = (x-1)+1=x

Tổng = (x+1)*SSH:2

210 = (x+1)*x:2

suy ra (x+1) *x = 210*2=420

          20*21=420

Vậy x=20

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

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

31 số x

31.x          +(30+1).30:2=1240

31.x          +465           =1240

31.x                            =1240-465

31.x                            =775

    x                             =775:31

    x                             =25

Vậy x=25

CHÚC BẠN HỌC TỐT

Thấy dãy trên có 31 số hạng

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

<=> x + x  + 1 + x + 2 + ... + x + 30 = 1240 

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

\(\Leftrightarrow31x+\frac{\left(30+1\right).30}{2}=1240\)

\(\Leftrightarrow31x=775\)

\(\Leftrightarrow x=25\)

\(x+\left(x+1\right)+\left(x+2\right)+...+\left(x+30\right)=1240\)

\(31x+\left(1+2+3+...+30\right)=1240\)

\(31x+\frac{\left(30+1\right).30}{2}=1240\)

\(31x+465=1240\)

\(\Leftrightarrow31x=775\)

\(\Leftrightarrow x=25\)

Vậy \(x=25\)