Let a, b, c \1, 2, 3, 4\. If the probability, that ax2+2 2 bx+c>0 for all x R, is mn, (m, n)=1, then m+n is equal to .
Answer
81
Solution
Total triples (a,b,c)=43=64.
∵ a>0, the expression is positive for all real x iff D<0:
D=8b2-4ac<0 ac>2b2
b=1: ac>2; pairs with ac 2 are (1,1),(1,2),(2,1) ⇒ 16-3=13
b=2: ac>8: (3,3),(3,4),(4,3),(4,4) ⇒ 4
b=3: ac>18, b=4: ac>32 ⇒ none (∵ ac 16)
Favourable =13+4=17 ⇒ probability = 1764
∴ m+n=17+64=81
Answer: 81