Of all nature's disasters, forest fires are often considered the most frightening. Moving at lightning speed, huge walls of flames can burn acres of land in just a few minutes. And although technology, including the use of fire-retardant chemicals, has greatly helped the fight against forest fires, they still do great damage.

    According to the National Interagency Fire Center, the fire season last year was the worst on record in terms of the number of fires and acres burned. There were 96. 385 fires and 9.873429 acres affected by fire in a year that was 125% more destructive(毁灭性的) than the10-year average. These fires cost the federal government $1.5 billion to fight, and this figure does not include the money spent by local and state governments as part of the effort.

    Fires in the southern states represented half of the national total last year with Texas, Oklahoma and Arkansas being home to a usually high amount of activity. Wildfire has also taken place in Western states such as California, Colorado and Alaska throughout the past 10 years.

    It is said that two-thirds of forest fires are started accidentally by people. Almost one quarter are purposely set, while lighting causes 10%.

    Forest fires can have advantageous effects. Charcoal(木炭) enriches soil and some plant species(种类) grow well after fire. The cones of the jack pine tree, for example, will not spread their seeds unless there is heat. Douglas fir trees grow best in open sunlight areas after fire.

已知sinα=$\frac{1}{3}$+cosα，且α∈（0，$\frac{\mathrm{\pi }}{2}$），则$\frac{\cos 2\alpha }{\sin \left(\alpha +\frac{\mathrm{\pi }}{4}\right)}$的值为____ 

①用洁净的铂丝蘸取溶液在无色火焰上灼烧，火焰呈黄色；

②把溶液分成2份，向一份溶液中加入淀粉溶液，再通入氯气，溶液呈蓝色；

③向另一份溶液中加入过量稀硫酸后，产生无色气体，再加氯化钡溶液后，产生白色沉淀．

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