Trigonometry! 12

Geometry Level 3

Given that 0 < α , β < π 4 0<\alpha,\beta<\dfrac {\pi}{4} such that cos ( α + β ) = 4 5 \cos (\alpha + \beta) = \dfrac {4}{5} and sin ( α β ) = 5 13 \sin (\alpha - \beta) = \dfrac {5}{13} holds true, find tan 2 α \tan 2\alpha .

65 15 \frac {65}{15} 25 26 \frac {25}{26} 56 33 \frac {56}{33} 32 15 \frac {32}{15}

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3 solutions

Using Pythagoras Theorem:-
S i n ( α + β ) = 3 5 , a n d C o s ( α β ) = 12 13 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 = 3 5 , S i n α C o s β C o s α S i n β = 5 13 C o s α C o s β S i n α S i n β = 4 5 , C o s α C o s β + S i n α S i n β = 12 13 2 S i n α C o s β = 3 5 + 5 13 = 64 65 . . . ( 1 ) , a n d 2 C o s α C o s β = 4 5 + 12 13 = 112 65 . . . . . . . ( 2 ) ( 1 ) ( 2 ) = t a n ( α ) = 64 112 = 4 7 t a n 2 α = 2 t a n ( α ) 1 t a n 2 α = 56 33 Sin(\alpha +\beta)=\dfrac{3}{5}, ~and ~~~~~~~~Cos(\alpha -\beta)=\dfrac{12}{13} \\Expanding~~\\Sin\alpha*Cos\beta+Cos\alpha*Sin\beta =\dfrac{3}{5} ,~~~Sin\alpha*Cos\beta-Cos\alpha*Sin\beta =\dfrac{5}{13} \\ Cos\alpha*Cos\beta-Sin\alpha*Sin\beta =\dfrac{4}{5} ,~~~Cos\alpha*Cos\beta+Sin\alpha*Sin\beta =\dfrac{12}{13}\\ \implies~2Sin\alpha*Cos\beta=\dfrac{3}{5}+ \dfrac{5}{13}=\dfrac{64}{65} ...(1) , ~and~~~~~\\\implies~2Cos\alpha*Cos\beta=\dfrac{4}{5}+\dfrac{12}{13}=\dfrac{112}{65}.......(2)\\\therefore~ \dfrac{(1)}{(2)}=tan(\alpha)=\dfrac{64}{112}=\dfrac{4}{7}\\\therefore~tan2\alpha=\dfrac{2tan(\alpha)}{1-tan^2\alpha }=~~~~\color{#D61F06}{\boxed{\dfrac{56}{33} } }

t a n 2 α = t a n [ ( α + β ) + ( α β ) ] tan2\alpha = tan[(\alpha + \beta) + (\alpha - \beta)]

t a n 2 α = t a n ( α + β ) + t a n ( α β ) 1 t a n ( α + β ) t a n ( α β ) tan2\alpha = \dfrac{tan(\alpha + \beta) + tan(\alpha - \beta)}{1 - tan(\alpha + \beta)tan(\alpha - \beta)}

From the given values we can find t a n ( α + β ) , t a n ( α β ) tan(\alpha + \beta),tan(\alpha - \beta)

sandeep Rathod - 6 years, 4 months ago

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Ya I followed same path.

shivamani patil - 5 years, 11 months ago
Chew-Seong Cheong
Jul 25, 2016

tan 2 α = sin 2 α cos 2 α = sin ( ( α + β ) + ( α β ) ) cos ( ( α + β ) + ( α β ) ) = sin ( α + β ) cos ( α β ) + cos ( α + β ) sin ( α β ) cos ( α + β ) cos ( α β ) sin ( α + β ) sin ( α β ) = 3 5 × 12 13 + 4 5 × 5 13 4 5 × 12 13 3 5 × 5 13 = 36 + 20 48 15 = 56 33 \begin{aligned} \tan 2 \alpha & = \frac {\sin 2 \alpha}{\cos 2 \alpha} \\ & = \frac {\sin ((\alpha + \beta) +(\alpha - \beta))}{\cos ((\alpha + \beta) +(\alpha - \beta))} \\ & = \frac {\sin(\alpha + \beta)\cos(\alpha - \beta) + \cos(\alpha + \beta)\sin(\alpha - \beta)}{\cos(\alpha + \beta)\cos(\alpha - \beta) - \sin(\alpha + \beta)\sin(\alpha - \beta)} \\ & = \frac {\frac 35 \times \frac {12}{13} + \frac 45 \times \frac {5}{13}}{\frac 45 \times \frac {12}{13} - \frac 35 \times \frac {5}{13}} = \frac {36+20}{48-15} = \boxed{\dfrac {56}{33}} \end{aligned}

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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