x + (x + 1) + (x + 2) + (x + 3) + ... + (x + 30) = 1240
(x + x + x + x + ... + x) + (0 + 1 + 2 + 3 + ... + 30) = 1240
31 số x 31 số hạng
31.x + (0 + 30).31:2 = 1240
31.x + 30.31:2 = 1240
31.x + 15.31 = 1240
31.(x + 15) = 1240
x + 15 = 1240 : 31
x + 15 = 40
x = 40 - 15
x = 25
x + ( x + 1 ) + ( x +2 ) + ( x+ 3) + ... + ( x + 30 ) = 1240
( x + x + x + x + ... + x ) + ( 1 + 2 + 3 + ... + 30 ) = 1240
31 x + 465 =1240
31x =775
x =25
x+(x+1)+(x+2)+(x+3)+...+(x+30)=1240
(30+1).x+(1+2+3+...+30)=1240
31.x+[(30+1).30:2]=1240
31.x+[31.30:2]=1240
31.x+465=1240
=>31.x=1240-465
=>31.x=775
=>x=775:31
=>x=25
Chúc bạn học giỏi nha!!!
K cho mik vs nhé làm gì
x + ( x + 1 ) + ( x +2 ) + ( x+ 3) + ... + ( x + 30 ) = 1240
( x + x + x + x + ... + x ) + ( 1 + 2 + 3 + ... + 30 ) = 1240
31 x + 465 =1240
31x =775
x =25
x + (x+1) + (x+2) + (x+3) + ...+ (x+30) = 1240
=> 31x + ( 1+ 2 + 3+ 4+ .... + 30) = 1240
Áp dụng công thức tính dãy số ta có :
1 + 2 + 3 + 4 + ... + 30 = \(\frac{\left[\left(30-1\right):1+1\right].\left(30+1\right)}{2}=\frac{30.31}{2}=15.31=465\)
=> 31x + 465 = 1240
=> 31x = 775
=> x = 25
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