I'm David．When I was 13,my parents decided that I should go to England to improve my English．I had never been to other countries before，so I felt a little worried．My parents prepared everything for my journey．My plane was due(预期)to leave at 9：30 a．m．and arrive in two hours．I phoned the English family who I would stay with and they told me they would wait for me at the airport(机场)．

Unluckily my plane to London was delayed (延误)．I tried to phone the English family again but I couldn't speak to them because they had already left home for the airport．

When I arrived in London．it was already 1：30p.m．I went to get my luggage(行李)but I couldn't find it．I became very worried．Luckily, the airport staff(工作人员)was very helpful and efficient．I didn't wait long-they found it 10 minutes later．

My English family were still waiting for me! I was so happy! Then my new life in London began．My English family were quite kind．I learned a lot of English because I spent most of the day talking in English．I played computer games with the little boy in my English family．I also taught him how to play chess．Sometimes I told him stories I learned from my family．

I missed my family during my stay in London．but I also feIt sad when my stay there came to an end．My English family held a party for me．I enjoyed it very much．I’ll never forget that summer in London! It is one of the most wonderful experiences I've ever had．

若关于 、 的二元一次方程组 $\{\begin{array}{c}3\text{x-}5\text{y=}2\text{a}\\ 2\text{x+}7\text{y=a-}18\end{array}$ 的解中x与y的值互为相反数，求 的值；

有一足够大的水池，在其水平池底竖直放置一段圆木．圆木可近似看作一个圆柱体，底面积0.8m²，高5m，密度0.7×10³kg/m³。（g=10N/kg）

（1）未向水池内注水时，圆木对池底的压力和压强分别为多大？

（2）向水池内缓慢注水，在水位到达1m时圆木受到水的浮力和圆木对池底的压力分别为多大？

（3）当向水池内注水深度达到4m时，圆木受到的浮力又为多大？