题目
How many ordered pairs of positive integers (m,n) satisfy GCD (m3,n2)=22∗32 and LCM (m2,n3)=24∗34∗56 where, GCD- Greatest common divisor and LCM- least common multiple
解析
有多少个有序正整数对\((m,n)\)满足\(GCD(m^{3},n^{2}) = 2^{2}\times3^{2}\)且\(LCM(m^{2},n^{3})=2^{4}\times3^{4}\times5^{6}\),其中\(GCD\)表示最大公约数,\(LCM\)表示最小公倍数
设\(m = 2^{a}\times3^{b}\times5^{c}\),\(n=2^{d}\times3^{e}\times5^{f}\)
根据\(GCD(m^{3},n^{2})=2^{2}\times3^{2}\),可得:
\(GCD(m^{3},n^{2}) = 2^{\min(3a, 2d)}\times3^{\min(3b,2e)} \times5^{\min(3c,2f)}\)
所以\(\min(3a,2d)=2\),\(\min(3b,2e) = 2\),\(\min(3c,2f)=0\)
根据\(LCM(m^{2},n^{3})=2^{4}\times3^{4}\times5^{6}\),可得:
\(LCM(m^{2},n^{3})=2^{\max(2a,3d)}\times3^{\max(2b,3e)}\times5^{\max(2c,3f)}\)
所以\(\max(2a,3d)=4\),\(\max(2b,3e)=4\),\(\max(2c,3f)=6\)
对于\(\min(3a,2d)=2\)和\(\max(2a,3d)=4\):
- 当\(3a = 2\)时,\(a=\frac{2}{3}\)(舍去,因为\(a\)是整数)
- 当\(2d=2\)即\(d = 1\)时,由\(\max(2a,3d)=4\)可得\(2a=4\),\(a = 2\)
- 当\(3a=4\)时,\(a=\frac{4}{3}\)(舍去)
- 当\(2d = 4\)即\(d=2\)时,由\(\min(3a,2d)=2\)可得\(3a=2\),\(a=\frac{2}{3}\)(舍去)
同理,对于\(\min(3b,2e)=2\)和\(\max(2b,3e)=4\)可得\(b = 2\),\(e=1\)或\(b=1\),\(e = 2\)
对于\(\min(3c,2f)=0\)和\(\max(2c,3f)=6\)可得\(c = 3\),\(f=2\)或\(c=2\),\(f = 3\)
\((a,b,c)\)和\((d,e,f)\)的组合有\((2,2,3)\)与\((1,1,2)\)以及\((2,2,3)\)与\((1,2,3)\)两种情况,即有序正整数对\((m,n)\)有\(2\)对,答案为B