Question

4. Cho A=11...111 ( 2018 số 1 ), B=111...13 ( 2017 số 1 )

 CMR ab+1 là số chính phương

Answer 1

\(ab+1=\underbrace{11....11}_{2018c/s1}.\underbrace{11....13}_{2017c/s1}+1\)

\(\Leftrightarrow ab+1=(\underbrace{11....10}_{2017c/s1}+1).(\underbrace{11....10}_{2017c/s1}+3)+1\)

\(\Leftrightarrow ab+1=\underbrace{11....10^2}_{2017c/s1}+4.\underbrace{11....10}_{2017c/s1}+3+1\)

\(\Leftrightarrow ab+1=\underbrace{11....10^2}_{2017c/s1}+4.\underbrace{11....10}_{2017c/s1}+4\)

\(\Leftrightarrow ab+1=(\underbrace{11....10}_{2017c/s1}+2)^2\) là số chính phương

Vậy...

C áp dụng hằng đẳng thức : \(x^2+2xy+y^2=\left(x+y\right)^2\)