解方程 $4\times2^{x+2}-5\times2^{2-x}=3$。答案保留 $3$ 位有效数字。

Answer all questions. You should use a calculator where appropriate.

You must show all necessary working clearly; no marks will be given for unsupported answers from a calculator.

Give non-exact numerical answers correct to 3 significant figures, or 1 decimal place for angles in degrees, unless a different level of accuracy is specified in the question.

请回答所有题目。适当时应使用计算器。必须清楚展示所有必要的计算过程；仅给出计算器答案而没有过程，不予评分。除非题目另有精度要求，非精确数值答案保留 3 位有效数字，以度为单位的角度保留 1 位小数。

令正数变量 $t=2^x$，把负指数写成倒数，化为二次方程。舍去负根后再取对数。

Let $t=2^x>0$. Since $2^{x+2}=4t$ and $2^{2-x}=4/t$,

The negative root is impossible because $t>0$. Hence