Explanation
$$AM\ge GM$$
$$\dfrac { { 4 }^{ x }+{ 4 }^{ 1-x } }{ 2 } \ge \sqrt { { 4 }^{ x }.{ 4 }^{ 1-x } } $$
$${ 4 }^{ x }+{ 4 }^{ 1-x }\ge 4$$
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