1.

If – x2 + 3x + 4 > 0, then which of the following is correct?(a) x∈ (– 1, 4) (b) x∈ [– 1, 4] (c) x∈ (– ∞, – 1) ∪ (4, ∞) (d) x∈ (– ∞, 1] ∪ [4, ∞)

Answer»

(a) x∈ (– 1, 4)

– x2 + 3x + 4 > 0 

⇒ x2 – 3x – 4 < 0 ⇒ (x – 4) (x + 1) < 0 

⇒ (x – 4) < 0, (x + 1) > 0 or (x – 4) > 0, (x + 1) < 0 

⇒ x < 4, x > – 1 or x > 4, x < – 1 

⇒ – 1 < x < 4.



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