f(x)=-acos2x-2根号3asinxcosx+2a+b的定义域为[0,pai/2]值域为[-5,1]求a和b
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f(x)=-acos2x-2根号3asinxcosx+2a+b的定义域为[0,pai/2]值域为[-5,1]求a和b
![f(x)=-acos2x-2根号3asinxcosx+2a+b的定义域为[0,pai/2]值域为[-5,1]求a和b](/uploads/image/z/18206671-31-1.jpg?t=f%28x%29%3D-acos2x-2%E6%A0%B9%E5%8F%B73asinxcosx%2B2a%2Bb%E7%9A%84%E5%AE%9A%E4%B9%89%E5%9F%9F%E4%B8%BA%5B0%2Cpai%2F2%5D%E5%80%BC%E5%9F%9F%E4%B8%BA%5B-5%2C1%5D%E6%B1%82a%E5%92%8Cb)
∵f(x)=-acos2x-√3asin2x+2a+b
=-2a(cos2x/2+√3/2*sin2x)+2a+b
=-2asin(2x+π/6)+2a+b
∵0≤x≤π/2∴π/6≤2x+π/6≤7π/6,
∴-1/2≤sin(2x+π/6)≤1,
当a>0,f(x)max=a+2a+b=3a+b=1,
f(x)min=-2a+2a+b=b=-5,
解得a=2,b=-5,
当a=0,不合题意,舍去.
当a<0,f(x)max=-2a+2a+b=b=1,
f(x)min=a+2a+b=3a+b=-5,
解得a=-2,b=1,
∴a=2,b=-5,或a=-2,b=1
=-2a(cos2x/2+√3/2*sin2x)+2a+b
=-2asin(2x+π/6)+2a+b
∵0≤x≤π/2∴π/6≤2x+π/6≤7π/6,
∴-1/2≤sin(2x+π/6)≤1,
当a>0,f(x)max=a+2a+b=3a+b=1,
f(x)min=-2a+2a+b=b=-5,
解得a=2,b=-5,
当a=0,不合题意,舍去.
当a<0,f(x)max=-2a+2a+b=b=1,
f(x)min=a+2a+b=3a+b=-5,
解得a=-2,b=1,
∴a=2,b=-5,或a=-2,b=1
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