$A=\left[1+\left(-2\right)\right]+\left[3+\left(-4\right)\right]+....+\left[2013+\left(-2014\right)+2015\right]$

$A=\left(-1\right)+\left(-1\right)+....+\left(-1\right)+2015\left(\text{1007 soˆˊ hạng }\left(-1\right)\right)=1008$

To: 

Subject: Rock climbing next Friday?

Hey

Are you free next Friday?

We are going to rock climbing on the mountain. I know you like outdoor sports so I think you will come with us.

We will meet at my home. It costs 20 dollars to rent tools to climb. Jane says it's really interesting and worth the money.

Don't worry, it's not dangerous because the school provides many safety equipments to wear.

Let me think if you want to come by Wednesday. I want to call the school to book equipment for mountain climbing the day after.

I hope you will come.

See you soon,