What is the common factors of 12 and 42 also 63 and 84
\(12|2 \\ 6 \ |2 \\ 3 \ |3 \\ 1 \\\\12=2^2*3 \\\\ 42|2\\21|3\\7\ |7\\1 \\\\42=2*3*7 \\\\ G.C.F(12;42)=2*3=6 \\\\\\\ 63|3 \\ 21|3 \\ 7 \ |7 \\ 1 \\\\ 63=3^2*7 \\\\ 84|2 \\ 42|2 \\ 21|3 \\ 7 \ |7 \\ 1 \\\\ 84=2^2*3*7 \\\\ G.C.F(63;84)=3*7=21\)
