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cos² (x + y) + sin² (x + y) = 1
(4/5)² + sin² (x + y) = 1
16/25 + sin² (x + y) = 1
sin² (x + y) = 1 - 16/25
sin² (x + y) = 9/25
sin (x + y) = √(9/25)
sin (x + y) = 3/5
tan (x + y) = sin (x + y) / cos (x + y)
= (3/5) / (4/5)
= 3/4
sin (x - y) = 5/13
sin² (x - y) + cos² (x - y) = 1
(5/13)² + cos² (x - y) = 1
25/169 + cos² (x - y) = 1
cos² (x - y) = 1 - 25/169
cos² (x - y) = 144/169
cos (x - y) = √(144/169)
cos (x - y) = 12/13
tan (x - y) = sin (x - y) / cos (x - y)
= (5/13) / (12/13)
= 5/12
tan [ (x + y) + (x - y) ] = [ tan (x + y) + tan (x - y) ] / (1 - tan (x + y) .tan (x - y)
tan (x + y + x - y) = (3/4 + 5/12) / (1 - 3/4 . 5/12)
tan 2x = (14/12) / (1 - 15/48)
tan 2x = (14/12) / (33/48)
tan 2x = 14/12 X 48/33
= 56/33
= 1 23/33