d: Ta có: ƯCLN(a,b)=2
=>\(\left\{{}\begin{matrix}a=2x\\b=2y\end{matrix}\right.\)
\(a\cdot b=120\)
=>\(2x\cdot2y=120\)
=>\(x\cdot y=30\)
mà x,y là các số nguyên dương
nên \(\left(x,y\right)\in\left\{\left(1;30\right);\left(2;15\right);\left(3;10\right);\left(5;6\right);\left(6;5\right);\left(10;3\right);\left(15;2\right);\left(30;1\right)\right\}\)
=>\(\left(a,b\right)\in\left\{\left(2;60\right);\left(4;30\right);\left(6;20\right);\left(10;12\right);\left(12;10\right);\left(20;6\right);\left(30;4\right);\left(60;2\right)\right\}\)
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