Bài 1: tìm x biết:

Question

Bài 1 Tìm x

1+3+5+......+x=1600

Answer 1

theo cong thu s=(soccuoi+so dau)*sososohang/2     

  suy ra (so dau+so cuoi)*sosohang=1600/2            

          (sodau +so cuoi)*sosohang=800

theo cong thuc tinh sosohang =(socuoi-so dau)/khoangcah+1

suy ra sosohang=(n-1)/2+1=n+1

suy ra n la le

n co the nam trong khoang tu 81 den 201

neu n = 81thi

ssh=(81-1)/2+1=41

tong1600=82*41/2=sai

n=83thi

ssh=82/2+1=42

tong=83*42/2=thap phan sai

n=85thi

ssh=84/2+1=43

tong =86*43/2=sai

n =87thi 

ssh=86/2+1=44

tong=88*44/2sai suy ra n <81</p>

neu n=79thi

ssh=78/2+1=40

tong=80*40/2=1600 dung 

suy ra x o bai ban =79