1/ a + b = 13
\(\Rightarrow a=2;b=11\)
2/ a + b = 19
\(\Rightarrow a=2;b=17\)
3/ a + b = 21
\(\Rightarrow a=2;b=19\)
4/ a + b = 31
\(\Rightarrow a=2;b=29\)
5/ a + b = 39
\(\Rightarrow a=2;b=37\)
Bài 21: tìm hai số nguyên tố a,b biết
`1) a+b =13`\(\Rightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}a=2\\b=11\end{matrix}\right.\\\left\{{}\begin{matrix}a=11\\b=2\end{matrix}\right.\end{matrix}\right.\)
`2)a +b =19``<=>`\(\left[{}\begin{matrix}\left\{{}\begin{matrix}a=17\\b=2\end{matrix}\right.\\\left\{{}\begin{matrix}a=2\\b=17\end{matrix}\right.\end{matrix}\right.\)
`3)a+b=21 ``<=>`\(\left[{}\begin{matrix}\left\{{}\begin{matrix}a=2\\b=19\end{matrix}\right.\\\left\{{}\begin{matrix}a=19\\b=2\end{matrix}\right.\end{matrix}\right.\)
`4)a+b=31`\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}a=2\\b=39\end{matrix}\right.\\\left\{{}\begin{matrix}a=39\\b=2\end{matrix}\right.\end{matrix}\right.\)
`5)a+b=39`\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}a=2\\b=37\end{matrix}\right.\\\left\{{}\begin{matrix}a=37\\b=2\end{matrix}\right.\end{matrix}\right.\)
