Oh my sun sin!

Geometry Level 2

sin ( A ) sin ( 60 A ) sin ( 60 + A ) = 1 x sin 3 A F i n d : ( 1.5 ) ( x ) = ? \sin { (A) } \sin { (60-A) } \sin { (60+A) } =\frac { 1 }{ x } \sin { 3A } \\ \\ Find:\\ (1.5)(x)\quad =\quad ?


The answer is 6.

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3 solutions

Daniel Liu
Jul 3, 2014

Equipped with the knowledge that this is an identity (since there is only one possible answer for x x ) we simply plug in a value for A A and solve for x x .

We plug in x = 3 0 x=30^{\circ} to get sin 30 sin 30 sin 90 = 1 x sin 90 \sin30\sin30\sin90=\dfrac{1}{x}\sin90

Solving gives 1 x = 1 4 x = 4 \dfrac{1}{x}=\dfrac{1}{4}\implies x=4

Thus the answer is 1.5 × 4 = 6 1.5\times 4=\boxed{6}

yeah man that's correct :)

joram otero - 6 years, 11 months ago
Mohamed El-Rayany
Jan 20, 2015

sin ( 60 A ) = 3 2 cos A 1 2 sin A sin ( 60 + A ) = 3 2 cos A + 1 2 sin A t h e n , sin A . sin ( 60 A ) . sin ( 60 + A ) = sin A . [ 3 2 cos A 1 2 sin A ] . [ 3 2 cos A + 1 2 sin A ] = sin A . [ 3 4 cos 2 A 1 4 sin 2 A ] b u t , cos 2 A = 1 2 [ 1 + cos A ] sin 2 A = 1 2 [ 1 cos A ] t h e n , = sin A . 3 8 ( 1 + cos 2 A ) 1 8 ( 1 cos 2 A ) = sin A . [ 3 8 + 3 8 cos 2 A 1 8 + 1 8 cos 2 A ] = 2 8 sin A + 4 8 sin A . cos 2 A b u t , sin A . cos 2 A = 1 2 [ sin 3 A sin A ] t h e n , = 2 8 sin A + 2 8 sin 3 A 2 8 sin A = 1 4 sin A S o , t h e a n s w e r i s x = 4 1.5 x = 6 \begin{matrix} \sin\ { (60-A) } =\frac { \sqrt { 3 } }{ 2 } \cos\ { A } -\frac { 1 }{ 2 } \sin\ { A } \\ \sin\ { (60+A) } =\frac { \sqrt { 3 } }{ 2 } \cos\ { A }+\frac { 1 }{ 2 } \sin\ { A } \\ then, \\ \sin\ { A }.\sin\ { (60-A) } .\sin\ { (60+A) } = \\ \sin\ { A }.\left[ \frac { \sqrt { 3 } }{ 2 } \cos\ { A }-\frac { 1 }{ 2 } \sin\ { A } \right] .\left[ \frac { \sqrt { 3 } }{ 2 } \cos\ { A } +\frac { 1 }{ 2 } \sin\ { A } \right] \\ =\sin\ { A } .\left[ \frac { 3 }{ 4 } \cos\ ^{ 2 }{ A } -\frac { 1 }{ 4 } \sin\ ^{ 2 }{ A } \right] \\ but, \\ \cos\ ^{ 2 }{ A } =\frac { 1 }{ 2 } \quad \left[ 1\quad +\quad \cos\ { A } \right] \\ \sin\ ^{ 2 }{ A } =\frac { 1 }{ 2 } \quad \left[ 1\quad -\quad \cos\ { A } \right] \\ then, \\ =\sin\ { A }.\frac { 3 }{ 8 } (1+\cos\ { 2 }{ A })\quad -\quad \frac { 1 }{ 8 } (1-\cos\ { 2 }{ A }) \\ =\sin\ { A } .\left[ \frac { 3 }{ 8 } +\frac { 3 }{ 8 } \cos\ { 2 } { A } \quad-\quad \frac { 1 }{ 8 } +\frac { 1 }{ 8 } \cos\ { 2 } { A } \right]\\=\frac { 2 }{ 8 } \sin\ { A } +\frac { 4 }{ 8 } \sin\ { A } .\cos\ { 2 } { A } \\ but, \\ \sin\ { A } .\cos\ { 2 } { A }=\frac { 1 }{ 2 } \left[ \sin\ { 3A } -\sin\ { A } \right] \\ then, \\ =\frac { 2 }{ 8 } \sin\ { A } +\frac { 2 }{ 8 } \sin\ { 3A } -\frac { 2 }{ 8 } \sin\ { A } =\frac { 1 }{ 4 } \sin\ { A } \\ So,\quad the\quad answer\quad is\quad x\quad =\quad 4 \\ 1.5x\quad =\quad \boxed { 6 } \end{matrix}

Pranjal Shukla
Jul 2, 2014

Since 4sin(60-A)sinAsin(60+A)=sin3A So 1.5*4=6.

How do you know that?

Calvin Lin Staff - 6 years, 11 months ago

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L H S = sin A sin ( 60 A ) sin ( 60 + A ) = sin ( A ) [ s i n 2 60 s i n 2 A ] = sin ( A ) [ 3 4 s i n 2 A ] = 3 sin ( A ) 4 s i n 3 A = 1 4 [ 3 sin ( A ) 4 s i n 3 A ] = 1 4 sin ( 3 A ) = R H S LHS=\sin { A } \sin { (60-A) } \sin { (60+A) } \\ \quad \quad \quad =\sin { (A) } \left[ { sin }^{ 2 }60-{ sin }^{ 2 }A \right] \\ \qquad =\sin { (A) } \left[ \frac { 3 }{ 4 } -{ sin }^{ 2 }A \right] \\ \qquad =\frac { 3\sin { (A) } }{ 4 } -{ sin }^{ 3 }A\\ \qquad =\frac { 1 }{ 4 } \left[ 3\sin { (A) } -{ 4sin }^{ 3 }A \right] \\ \qquad =\frac { 1 }{ 4 } \sin { (3A) } \quad =\quad RHS

i hope this helps :)

joram otero - 6 years, 11 months ago

Remember to give justification of whatever you write in your solution

Dinesh Chavan - 6 years, 11 months ago

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