21. Butun sonlarning ko‘paytmasi ishorasini aniqlash

Ushbu 1 · 0 ifodani ishorasi manfiy bo‘lsa YES so’zini, musbat bo‘lsa NO so‘zini, agar 0 ga teng bo‘lsa NULL so‘zini kiriting.

Masalan:

-a · (-b) = ab = NO
-1 · (ab) = -ab = YES
-1 · a · 0 = 0 = NULL
a, b ∈ ℕ+