保护环境，提倡“低碳生活”，是我们都应关注的社会问题。
⑴目前，一些汽车已改用压缩天然气（CNG）做燃料，以减少对空气污染。
①天然气（主要成分为CH₄）完全燃烧的化学方程式为____ ；
②从下表数据分析，与煤相比，用天然气做燃料的优点是____ 。

⑵生活中少买一件不必要的衣服可以节约2.5 kg标准煤（已知标准煤的平均含碳量为72%），则相应减排二氧化碳____ kg。
⑶为限制二氧化碳等温室气体的排放量，科学家采取“组合转化”技术，将二氧化碳和氢气在催化剂和加热条件下转化为一种重要的化工原料乙烯（C₂H₄）和水，该反应化学方程式为____ 。
⑷某地区赤铁矿（主要成分是Fe₂O₃）和黄铁矿（主要成分是FeS₂）储量丰富。若从保护环境的角度考虑，当地炼铁厂应选用的铁矿石是____ 。为了提高铁矿石的利用率，常将块状铁矿石____ ，该高炉炼铁的化学方程式为____ 。

已知： $x=\sqrt{3}+1$ ， $y=\sqrt{3}-1$ ，求 $\frac{x^2-2xy+y^2}{x^2-y^2}$ 的值．

    AROUND HANGZHOU is a special news program in English on HTV-1. It's about the city and lasts for 8 minutes. You can watch the program at 22:27 p.m. every Sunday and watch it again at 7:30 a.m. and 15:35 p.m. on Monday.
    Tel: 85195197           Add: 310004,139 North Ring Road, Hangzhou
    HANGZHOU WEEKLY is to help foreigners understand more about the city. It's a news edition in English in Hangzhou. It usually appears on the 48th edition of the city's most popular newspaper CITY EXPRESS every Thursday.
    Tel: 85051699            Add: 310041, 218 Tiyuchang Road, Hangzhou.
    IN TOUCH is an English language magazine. Turn the pages and inside you'll find passages on history, art, music, sports, travel and food of Zhejiang. At the end of every month you can get the magazine in hotels, restaurants and cafes all around Zhejiang without paying.
    Tel: 87630035 87633103    Add: 310007, 5 Huanglong Road, Hangzhou

如图是小麦遗传育种的一些途径．请回答下列问题：