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优先处理条件更多的函数(2022全国乙,12)

2023-03-01 21:06 作者:数学老顽童  | 我要投稿

(2022全国乙,12)已知函数f%5Cleft(%20x%20%5Cright)%20g%5Cleft(%20x%20%5Cright)%20的定义域均为%5Cmathbf%7BR%7D,且f%5Cleft(%20x%20%5Cright)%20%2Bg%5Cleft(%202-x%20%5Cright)%20%3D5g%5Cleft(%20x%20%5Cright)%20-f%5Cleft(%20x-4%20%5Cright)%20%3D7.若y%3Dg%5Cleft(%20x%20%5Cright)%20的图象关于直线x%3D2对称,g%5Cleft(%202%20%5Cright)%20%3D4,则%5Csum_%7Bk%3D1%7D%5E%7B22%7D%7Bf%5Cleft(%20k%20%5Cright)%7D%3D(    )

A.-21

B.-22

C.-23

D.-24

解:在f%5Cleft(%20x%20%5Cright)%20%2Bg%5Cleft(%202-x%20%5Cright)%20%3D5中,

x替换为x-4,可得

f%5Cleft(%20x-4%20%5Cright)%20%2Bg%5Cleft(%206-x%20%5Cright)%20%3D5

再与g%5Cleft(%20x%20%5Cright)%20-f%5Cleft(%20x-4%20%5Cright)%20%3D7联立,

消去f%5Cleft(%20x-4%20%5Cright)%20,可得

%5Ccolor%7Bred%7D%7Bg%5Cleft(%20x%20%5Cright)%20%2Bg%5Cleft(%206-x%20%5Cright)%20%3D12%7D……①

y%3Dg%5Cleft(%20x%20%5Cright)%20的图象关于点%5Ccolor%7Bred%7D%7B%5Cleft(%203%2C6%20%5Cright)%20%7D中心对称

因为y%3Dg%5Cleft(%20x%20%5Cright)%20的图象关于直线x%3D2对称,

所以%5Ccolor%7Bred%7D%7Bg%5Cleft(%20x%20%5Cright)%20%3Dg%5Cleft(%204-x%20%5Cright)%20%7D……②

结合①、②可知

g%5Cleft(%204-x%20%5Cright)%20%2Bg%5Cleft(%206-x%20%5Cright)%20%3D12

在上式中,将x替换为4-x,可得

%5Ccolor%7Bred%7D%7Bg%5Cleft(%20x%20%5Cright)%20%2Bg%5Cleft(%20x%2B2%20%5Cright)%20%3D12%7D……%5Coplus%20

半周期为%5Ccolor%7Bred%7D%7B2%7D

再将x替换为x%2B2,可得

g%5Cleft(%20x%2B2%20%5Cright)%20%2Bg%5Cleft(%20x%2B4%20%5Cright)%20%3D12……%5Cotimes%20

结合%5Coplus%20%5Cotimes%20可知

%5Ccolor%7Bred%7D%7Bg%5Cleft(%20x%2B4%20%5Cright)%20%3Dg%5Cleft(%20x%20%5Cright)%20%7D周期为%5Ccolor%7Bred%7D%7B4%7D

在①中,

x%3D3,可得g%5Cleft(%203%20%5Cright)%20%2Bg%5Cleft(%203%20%5Cright)%3D12

解得%5Ccolor%7Bred%7D%7Bg%5Cleft(%203%20%5Cright)%20%3D6%7D

x%3D4,可得g%5Cleft(%204%20%5Cright)%20%2Bg%5Cleft(%202%20%5Cright)%20%3D12

g%5Cleft(%204%20%5Cright)%20%2B4%3D12

解得%5Ccolor%7Bred%7D%7Bg%5Cleft(%204%20%5Cright)%20%3D8%7D

在②中,

x%3D1,可得%5Ccolor%7Bred%7D%7Bg%5Cleft(%201%20%5Cright)%20%3D%7Dg%5Cleft(%203%5Cright)%20%3D%5Ccolor%7Bred%7D%7B6%7D.

g%5Cleft(%20x%20%5Cright)%20-f%5Cleft(%20x-4%20%5Cright)%20%3D7中,

x替换为x%2B4,可得

g%5Cleft(%20x%2B4%20%5Cright)%20-f%5Cleft(%20x%20%5Cright)%20%3D7

%20f%5Cleft(%20x%20%5Cright)%20%3Dg%5Cleft(%20x%2B4%20%5Cright)-7

%5Ccolor%7Bred%7D%7B%20f%5Cleft(%20x%20%5Cright)%20%3Dg%5Cleft(%20x%20%5Cright)-7%7D.

所以

%5Cbegin%7Baligned%7D%0A%09%5Csum_%7Bk%3D1%7D%5E%7B22%7D%7Bf%5Cleft(%20k%20%5Cright)%7D%26%3D%5Csum_%7Bk%3D1%7D%5E%7B22%7D%7Bg%5Cleft(%20k%20%5Cright)%7D-7%5Ctimes%2022%5C%5C%0A%09%26%3D5%5Csum_%7Bk%3D1%7D%5E4%7Bg%5Cleft(%20k%20%5Cright)%7D%2B%5Csum_%7Bk%3D1%7D%5E2%7Bg%5Cleft(%20k%20%5Cright)%7D-154%5C%5C%0A%09%26%3D5%5Ctimes%20%5Cleft(%206%2B4%2B6%2B8%20%5Cright)%20%2B%5Cleft(%206%2B4%20%5Cright)%20-154%5C%5C%0A%09%26%3D%5Ccolor%7Bred%7D%7B-24%7D%5C%5C%0A%5Cend%7Baligned%7D

D.


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