Question

Tìm \(x\in N\), biết: 

                          \(2+4+6+8+10+...+2x=156\) 

Answer 1

=> 2.( 1 + 2 + 3 + .... + x ) = 156

=> 2.{ [ x.( x + 1 ) ] : 2 } = 156

=> [ x.( x + 1 ) ] : 2 = 156 : 2

=> x.( x + 1 ) = 156 = 12.( 12 + 1 )

=> x .( x + 1 ) = 12.( 12 + 1 )

=> x = 12

Answer 2

2.1+2.2+2.3+2.4+....+2.x

=2.(1+2+3+4+.....+x)=156

1+2+3+4+.........+x=78

=> x=12 nha!

Answer 3

=> 2 ( 1 + 2 + 3 +.................... + x ) = 156

= 1 + 2 + 3 + 4 + ....... + x = 78

=> x = 2

Answer 4

x=12 nha bn