✔ 最佳答案
case (i), a, c are different sign, roots should be real.
case (ii), a, c are same sign,
b^2 - 4ac >= 0
b^2 >= 4ac
b^2 >= K where K is a positive real number
by the meaning of infinity, i guess (not sure), the probability = 1/2
thus, the required probability
= P(a, c are different sign) + P(a, c are same sign and b^2 >= K)
= 1/2 + (1/2)(1/2)
= 3/4