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问题: 函数问题

已知函数f(x)=cos(ax-b)(a>0,0≤b≤π)是R上的奇函数,其图像过(2π/3,1)点,并在(0,π/10)上是单调增函数,求a和b的值

解答:

f(x)=cos(ax-b)(a>0,0≤b≤π)是R上的奇函数
==>f(0)=0
====>cos(-b)=0
0≤b≤π
====>b=π/2

过(2π/3,1)点 ======>(2π/3)a-π/2 =2kπ (k是整数)
=====>a=3k+3/4 ...........................(1)
a>0 ====>k>=0 ..............(2)

x在(0,π/10)上是单调增函数
===>
(-π/2 , aπ/10 -π/2)在(-π/2 ,0)之内
(3k+3/4)*π/10 -π/2<=0
代入(1),
===>6k+3/2<=10
根据(2)
====>k=0,或1
===>a=3/4或15/4

所以,a=3/4或15/4,b=π/2