Given that 0 < α , β < 4 π such that cos ( α + β ) = 5 4 and sin ( α − β ) = 1 3 5 holds true, find tan 2 α .
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t a n 2 α = t a n [ ( α + β ) + ( α − β ) ]
t a n 2 α = 1 − t a n ( α + β ) t a n ( α − β ) t a n ( α + β ) + t a n ( α − β )
From the given values we can find t a n ( α + β ) , t a n ( α − β )
tan 2 α = cos 2 α sin 2 α = cos ( ( α + β ) + ( α − β ) ) sin ( ( α + β ) + ( α − β ) ) = cos ( α + β ) cos ( α − β ) − sin ( α + β ) sin ( α − β ) sin ( α + β ) cos ( α − β ) + cos ( α + β ) sin ( α − β ) = 5 4 × 1 3 1 2 − 5 3 × 1 3 5 5 3 × 1 3 1 2 + 5 4 × 1 3 5 = 4 8 − 1 5 3 6 + 2 0 = 3 3 5 6
Well, I tried a long method. I dont have time. sry, sin(2α) = sin[(α+ß)-(α-ß)] , cos(α-ß) = 12/13, sin(α-ß) = 3/5 Similarly, do that for cos(2α) And youll get after proper simplifications. Very easy. Please ask if explanation needed, but miene is a very long one :D
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Using Pythagoras Theorem:-
S i n ( α + β ) = 5 3 , a n d C o s ( α − β ) = 1 3 1 2 E x p a n d i n g S i n α ∗ C o s β + C o s α ∗ S i n β = 5 3 , S i n α ∗ C o s β − C o s α ∗ S i n β = 1 3 5 C o s α ∗ C o s β − S i n α ∗ S i n β = 5 4 , C o s α ∗ C o s β + S i n α ∗ S i n β = 1 3 1 2 ⟹ 2 S i n α ∗ C o s β = 5 3 + 1 3 5 = 6 5 6 4 . . . ( 1 ) , a n d ⟹ 2 C o s α ∗ C o s β = 5 4 + 1 3 1 2 = 6 5 1 1 2 . . . . . . . ( 2 ) ∴ ( 2 ) ( 1 ) = t a n ( α ) = 1 1 2 6 4 = 7 4 ∴ t a n 2 α = 1 − t a n 2 α 2 t a n ( α ) = 3 3 5 6