Let the domain of the function f(x)=cos−1(3x−74x+5) be [α,β] and the domain of g(x)=log2(2−6log27(2x+5)) be (γ,δ). Then ∣7(α+β)+4(γ+δ)∣ is equal to ______
Answer: 96
Solution
For f, require −1≤3x−74x+5≤1. The inequality 3x−74x+5≤1 reduces to 3x−7x+12≤0, giving −12≤x<37, while 3x−74x+5≥−1 reduces to 3x−77x−2≥0, giving x≤72 or x>37. Their intersection is [−12,72], so α=−12,β=72. For g, need 2x+5>0 and 2−6log27(2x+5)>0; the latter gives log27(2x+5)<31, i.e. 2x+5<271/3=3, so x<−1. With x>−25, the domain is (−25,−1), so γ=−25,δ=−1. Then 7(α+β)+4(γ+δ)=7(−782)+4(−27)=∣−82−14∣=96.