Question

1+3+5+...+x=1600. Tìm x

Answer 1

x=79

vì 1+3+5+..+79=1600

Answer 2

Từ 1→x có:(x-1):2+1

 Do đó ta có:1+3+5+...+x=1600

<=>[(x+1).(x-1)/2+1]:2=1600

<=>(x+1).(x-1)/2+1=1600.2=3200

<=>(x+1).(x-1)/2+2/2=3200

<=>(x+1).(x-1+2)/2=3200

<=>(x+1).(x+1)/2=3200

<=>(x+1)^2=3200.2=6400

<=>x+1=80=-80

<=>x=79 hoặc x=-81