若a、b 、c都是有理数，且|a-1|+|b+2|+|c-4|= 0，求 a+|b|+c的值。
解：原式=7。
Directions:
Write an essay of 160--200 words based on the following drawing. In your essay, you should
1) describe the picture briefly,
2) explain its intended meaning, and then
3) support your view with an example/examples.
You should write neatly on ANSWER SHEET 2.
噪声与振动测量中较常使用的滤波器是（）和恒定百分比带宽滤波器两类。
A、恒定带宽滤波器
B、窄带滤波器
C、带阻滤波器
D、以上都是
