怎么证明:函数 y = f (x)的图像关于点A (a ,b)对称的充要条件是f (x) + f (2a-x) = 2b?
问题描述:
怎么证明:函数 y = f (x)的图像关于点A (a ,b)对称的充要条件是f (x) + f (2a-x) = 2b?
答
证明:
必要性
设点P(x ,y)是y = f (x)图像上任一点,
∵点P( x ,y)关于点A (a ,b)的对称点P‘(2a-x,2b-y)也在y = f (x)图像上,
∴ 2b-y = f (2a-x)
即y + f (2a-x)=2b故f (x) + f (2a-x) = 2b,
必要性得证.
充分性
设点P(x0,y0)是y = f (x)图像上任一点,则y0 = f (x0)
∵ f (x) + f (2a-x) =2b
∴f (x0) + f (2a-x0) =2b,
即2b-y0 = f (2a-x0) .
故点P‘(2a-x0,2b-y0)也在y = f (x) 图像上,而点P与点P‘关于点A (a ,b)对称,
充分性得征.