tìm hai số tự nhiên biết rằng ƯCLN(a,b)=20
BCNN(a,b)=700
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a) goi hai so la a ; b va a >b
vi UCLN(a,b)=18=>a=18k ; b=18q (trong do UCLN (k,q)=1 va k>q)
=>a+b=162
18k+18q =162
18(k+q)=162
k+q=9
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21453
52542000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 | 542454550212.100000000000000000000000000000000000000000000000000000000000000000000000000000 |
Ta có: \(a.b=ƯCLN\left(a,b\right).BCNN\left(a,b\right)\)
\(\Rightarrow a.b=20.700\)
\(\Rightarrow a.b=14000\)
Vì \(ƯCLN\left(a,b\right)=20\Rightarrow\hept{\begin{cases}a=20.m\\b=20.n\end{cases};\left(m,n\right)=1;m,n\in N}\)
Thay \(a=20.m\), \(b=20.n\) vào \(a.b=14000\), ta có:
\(20.m.20.n=14000\)
\(\Rightarrow\left(20.20\right).\left(m.n\right)=14000\)
\(\Rightarrow400.\left(m.n\right)=14000\)
\(\Rightarrow m.n=14000\div400\)
\(\Rightarrow m.n=35\)
Vì m và n nguyên tố cùng nhau
\(\Rightarrow\) Ta có bảng giá trị:
Vậy các cặp (a, b) cần tìm là:
(20; 700); (700; 20); (100; 140); (140; 100).