Question

Tìm x thuộc N biết 2+4+6+..........+2x=210

Answer 1

2+4+6+8+...+2x=210

=>2.1+2.2+2.3+2.4+...+2.x=210

=>2.(1+2+3+4+...+x)=210

=>1+2+3+4+...+x=210:2

=>1+2+3+4+...+x=105

=>x=14

Answer 2

2+4+6+8+...+2x=210

=>2.1+2.2+2.3+2.4+...+2.x=210

=>2.(1+2+3+4+...+x)=210

=>1+2+3+4+...+x=210:2

=>1+2+3+4+...+x=105

=>x=14