①In the double-stranded DNA,

A=T, C=G

A+C=T+G (equation plus equation); or A+G=T+C,

That is, the sum of any two non-complementary bases is equal

②Between DNA double strands,

A1=T2, C1=G2, G1=C2, T1 ＝A2

(A1＋C1)/(G1＋T1)＝(T2＋G2)/(C2＋A2)

or (A1＋G1)/(T1＋C1)＝(T2＋C2)/(A2＋G2)