(1)
cos(π/4+α)-cos(π/4-α)
= [cos(π/4)cosα-sin(π/4)sinα]-[cos(π/4)cosα+sin(π/4)sinα]
= -2sin(π/4)sinα
= -√2sinα
(2)
cos(α+β)cosβ+sin(α+β)sinβ
= cos[(α+β)-β]
= cosα
(1)
cos(π/4+α)-cos(π/4-α)
= [cos(π/4)cosα-sin(π/4)sinα]-[cos(π/4)cosα+sin(π/4)sinα]
= -2sin(π/4)sinα
= -√2sinα
(2)
cos(α+β)cosβ+sin(α+β)sinβ
= cos[(α+β)-β]
= cosα