Question 77
If the real numbers $a, b, c, d$ are such that $0 \lt a, b, c, d \lt 1$, show that
$$
1 + a b + b c + c d + d a + a c + b d \gt a + b + c + d
$$
$$
1 + a b + b c + c d + d a + a c + b d \gt a + b + c + d
$$
MathNet 2008-part-02 — 165 problems (EN–VI) Question 77
Math Olympiad Vault · MathNet 2008-part-02 — 165 problems (EN–VI)