若\({y_1}\left( x \right), {y_2}\left( x \right)\)都是\(y' + P\left( x \right)y = Q\left( x \right)\)的特解，且 \({y_1}\left( x \right), {y_2}\left( x \right)\) 线性无关，则通解可表为\(y\left( x \right) = {y_1}\left( x \right) + C\left[ { { y_1}\left( x \right) - {y_2}\left( x \right)} \right]\)。

‍The order of using folks and knives in western dinners is _____________.‌ A: from left to right B: from right to left C: from inside to outside D: from outside to inside

设\( A,B \) 为方阵，则 \( \left| {AB} \right| = \,\left| A \right|\,\left| B \right| \)。（ ）

设\( A,B \)均为\( n \)阶方阵，则必有（ ） A: \( \left| {A + B} \right| = \left| A \right| + \left| B \right| \) B: \( AB = BA \) C: \( \left| {AB} \right| = \left| {BA} \right| \) D: \( {\left( {A + B} \right)^{ - 1}} = {A^{ - 1}} + {B^{ - 1}} \)

lesson 18 Directions: in this part, you are going to listen to five transmissions or short exchanges and choose the correct answer from four choices. A: 9 miles 6 o’clock 9000feet fast moving from left to right B: 9 miles 6 o’clock 9000feet crossing from right to left C: 6 miles 9 o’clock 9000feet fast moving from right to left D: 6 miles 9 o’clock 9000feet crossing from left to right