Oxford University MAT 2017 Solved Papers

Question : 2 of 10

Marks: +1, -0

The minimum value achieved by the function

f(x)=9 \cos ^4 x-12 \cos ^2 x+7

equals

Solution:

Substitute

γ=\cos ^2(x)

and solve the resulting quadratic in

γ

(noting that

0 ≤ γ ≤ 1

). The answer is (a).

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