S=ab
S1=1/2*a/3*2b/3=ab/9=S/9
S2=1/2*b/3*2a/3 =ab/9=S/9
S(PQRS) = S(ABCD) -2(S1+S2) =S -2(S/9+S/9)=S-4/9*S=9/9*S-4/9*S=5S/9
ответ 4) 5S/9
S=ab
S1=1/2*a/3*2b/3=ab/9=S/9
S2=1/2*b/3*2a/3 =ab/9=S/9
S(PQRS) = S(ABCD) -2(S1+S2) =S -2(S/9+S/9)=S-4/9*S=9/9*S-4/9*S=5S/9
ответ 4) 5S/9