下列各命题中正确的命题是
①“若$a,b$都是奇数，则$a+b$是偶数”的逆否命题是“若$a+b$不是偶数，则$a,b$都不是奇数”；
② 命题 “$\exists x\in R,x^2+1>3x$”的否定是“$\forall x\in R,x^2+1\le 3x$” ；
③ “函数$f\left(x\right)={\cos }^{2}ax-{\sin }^{2}ax$的最小正周期为$\pi$” 是“$a=1$”的必要不充分条件；
④“平面向量$\overrightarrow{a}$与$\overrightarrow{b}$的夹角是钝角”的充分必要条件是$\overrightarrow{a}·\overrightarrow{b}<0$ ．

Mum asked me to avoid    out alone at night．    

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Music teachers wanted for

No. 1Middle School

　　We want two music teachers for our school Music Club. You need to be able to play the guitar or the violin and be good with kids.

Time: 3: 20 p. m. —4: 20 p. m. Monday to Friday every week.

Pay: 100 yuan an hour.

Phone number: 372-3966

Email address: No. 1 middleschoolmc@126. com

Address: 326 Bridge Street. (Take No. 121 Bus. )

Art teacher wanted

　　Can you paint? Can you draw? Do you like kids and want to be with them? Then you can be in Yixiu Art Club.

Time: 8: 00 a. m. —10: 00 a. m. Saturday and Sunday every week.

Pay: 80 yuan an hour.

Phone number: 693-8925

Email address: yixiuartclub @163. com

Address: 165 Zhangshan Road. (Take No. 212 Bus. )