如图（甲）为小型旋转电枢式交流发电机的原理图，其矩形线圈在匀强磁场中绕垂直于磁场方向的固定轴OO′按如图所示方向匀速转动，线圈的匝数n=100、电阻r=10Ω，线圈的两端经集流环与电阻R连接，电阻R=90Ω，与R并联的交流电压表为理想电表．在t=0时刻，线圈平面与磁场方向平行，穿过每匝线圈的磁通量Φ随时间t按图（乙）所示正弦规律变化．（取π=3.14）求：

阅读下面材料，在空白处填入适当的内容（不多于1个单词）或括号内单词的正确形式。

    Nowadays, more and more schools in China have rules making students wear school uniforms to school. A lot of Chinese students complain about____ (wear) their school uniforms every day. But do American students get ____(annoy) about their uniforms, too? American high schools usually have a dress code(衣着标准), ____ is about requirements for students' dressing. Boys at school must wear clean jackets every day. Girls are ____ (luck) than boys, for they have more flexible____ (choose) than boys. They can either dress similarly to the boys ____wear a dress. In general, it takes a student 10-15 minutes every morning ____ (dress) up for class. As students do in China, plenty of American students also have their complaints about school uniforms. What if students really dislike the dress code and want to get rid of it? Instead of getting punished, ____ (actual) there're ways to do that. At my school, "dress down" tickets ____ (sell) on school days. If students are willing to buy a ticket, they don't need to wear school uniforms the following day. I have a strong  ____ (believe) that one day Chinese students can buy these tickets, too.

①“ $mn\text{=}nm$ ”类比得到“ $a\cdot b=b\cdot a$ ” ；

②“ $\left(m+n\right)t\text{=}mt+nt$ ”类比得到“ $(a+b)\cdot c=a\cdot c+b\cdot c$ ” ；

③“ $\left(m\cdot n\right)t\text{=}m\left(n\cdot t\right)$ ”类比得到“ $(a\cdot b)\cdot c=a\cdot (b\cdot c)$ ” .

以上式子中，类比得到的结论正确的个数是（   ）