Let "a,b\\in H\\cap K". Then "a,b\\in H" and "a,b\\in K". Since "(H,*)" and "(K,*)" are subgroups of
"(G,*)", "a*b\\in H" and "a*b\\in K". By analogy for the inverse element "a^{-1}" of the element "a" in group "G" we have that "a^{-1}\\in H" and "a^{-1}\\in K". Therefore, "a*b, a^{-1}\\in H\\cap K." So, we conclude that "(H\\cap K, *)" is a subgroup of "(G,*)."
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