A calculus problem by Aziz Alasha

Calculus Level 5

Find the minimum value of x 2 + 2 x + 2 x^2 + 2^x + 2 to 3 decimal places.


The answer is 2.902.

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1 solution

Tom Engelsman
Dec 6, 2016

If f ( x ) = x 2 + 2 x + 2 f(x) = x^2 + 2^x + 2 , then f ( x ) = 2 x + 2 x l n ( 2 ) = 0 x + 2 x 1 l n ( 2 ) = 0 2 x l n ( 2 ) = x . f'(x) = 2x + 2^x \cdot ln(2) = 0 \Rightarrow x + 2^{x-1} \cdot ln(2) = 0 \Rightarrow 2^x \cdot ln(\sqrt{2}) = -x. This exponential curve and line must intersect in the second quadrant of the x y xy- plane. At x = 1 x = -1 we obtain 2 1 l n ( 2 ) 0.1733 2^{-1} \cdot ln(\sqrt{2}) \approx 0.1733 for the LHS and 1 1 for the RHS, which implies the critical point must lie within x ( 1 , 0 ) . x \in (-1,0). After some trial-&-error I find the critical value to be x 0.2845 x \approx -0.2845 , and f ( x ) = 2 + 2 x ( l n ( 2 ) ) 2 > 0 f''(x) = 2 + 2^{x} \cdot (ln(2))^2 > 0 for all real x x . So the minimum value of f ( x ) f(x) computes to ( 0.2845 ) 2 + 2 0.2845 + 2 2.904. (-0.2845)^2 + 2^{-0.2845} + 2 \approx 2.904.

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