Question

x + (x + 1) + (x + 2) + (x + 3) +...+ (x +20) = 420

Answer 1

\(...\Rightarrow\left[\left(x+20-x\right):1+1\right].\left(x+x+20\right):2=420\)

\(\Rightarrow21.\left(x+x+20\right)=420.2\)

\(\Rightarrow21.\left(2x+20\right)=840\)

\(\Rightarrow\left(x+10\right)=840:\left(21.2\right)\)

\(\Rightarrow x+10=20\)

\(\Rightarrow x=10\)

Answer 2

Đề bài 

( = ) ( ( x + 20 - x ) : 1 + 1 ) . ( x + x + 20 ) : 2 = 420

( = ) 21 . ( x + x + 20 ) = 420 . 2

( = ) 21 . ( 2x + 20 ) = 840

( = ) ( x + 10 ) = 840 : ( 21 . 2 )

( = ) x + 10 = 20 

( = ) x = 10

Answer 3

X x 20+(1+2+3+...+20)=420

X x 21+ 210=420

X x 21=420-210

X x 21= 210

      X=210:21

     X=10

Chúc bạn học tốt :D

 

Answer 4

x + (x + 1) + (x + 2) + (x + 3) + ... + (x + 20) = 420

21x + 20 × 21 : 2 = 420

21x + 210 = 420

21x = 420 - 210

21x = 210

x = 210 : 21

x = 10