A positive attitude is very important as it will make our life easier. In life, not everything goes as ____(smooth) as we want. There will be times when we will face problems and difficulties. The most ____(challenge) part is ____we react when we experience problems and difficulties.

    Instead of being discouraged, those who have a positive attitude ____ (treat) each failure as a lesson. And they will be inspired to achieve the goal.____those who do not have the positive attitude will see problems and difficulties ____ a different way. With different attitudes, the result will be quite different. Since every one of us ____ (possess) the same amount of time, why don't we just make the best ____the same amount of time with a positive attitude? Life itself is already hard. It will be much ____(good) if we have a positive attitude which will make our life happier and easier.

    If you learn to see the bright side of life when you ____(surround) by difficulties, you'll be building your positive attitude fast.

材料一：春耕夏耘，秋获冬藏……勤苦如此，尚复被水旱之灾，急政暴赋，赋敛不时，朝令而暮改。当具有者半贾而卖，亡者取倍称之息。于是有卖田宅鬻子孙以偿债者矣。

——摘自（西汉）晁错《论贵粟疏》

材料二：北宋苏洵说：“耕者之田资于富民……而田之所入，已得其半，耕者得其半。”到了明清时期，多实行定额地租，佃农所交地租，多的占到收获物的百分之八十。

材料三：上自绅富，下至委巷工贾胥吏之俦，赢十百金，即莫不志在良田。”

——淸•陶煦《租核•推原》

请回答：

选做题（从下面两题中任选一题，如果做了两题的，只按第（1）题评分）

（1）比较大小：$\sqrt{13}$-$\sqrt{11}$____ $\sqrt{15}$-$\sqrt{13}$

（2）计算：sin40°•cos40°﹣tan50°≈____ （结果保留三个有效数字）．