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Verify the following identity:
sin((3 x)/2) (sin(x))/(sin(x/2)) = sin(x) + sin(2 x)

Multiply both sides by sin(x/2):
sin(x) sin((3 x)/2) = ^?sin(x/2) (sin(x) + sin(2 x))

sin(x) sin((3 x)/2) = 1/2 (cos(x - (3 x)/2) - cos(x + (3 x)/2)) = 1/2 (cos(-x/2) - cos((5 x)/2)):
(cos(-x/2) - cos((5 x)/2))/(2) = ^?sin(x/2) (sin(x) + sin(2 x))

Use the identity cos(-x/2) = cos(x/2):
(cos(x/2) - cos((5 x)/2))/(2) = ^?sin(x/2) (sin(x) + sin(2 x))

sin(x/2) (sin(x) + sin(2 x)) = sin(x/2) sin(x) + sin(x/2) sin(2 x):
(cos(x/2) - cos((5 x)/2))/(2) = ^?sin(x/2) sin(x) + sin(x/2) sin(2 x)

sin(x/2) sin(x) = 1/2 (cos(x/2 - x) - cos(x/2 + x)) = 1/2 (cos(-x/2) - cos((3 x)/2)):
(cos(x/2) - cos((5 x)/2))/(2) = ^?(cos(-x/2) - cos((3 x)/2))/(2) + sin(x/2) sin(2 x)

Use the identity cos(-x/2) = cos(x/2):
(cos(x/2) - cos((5 x)/2))/(2) = ^?(cos(x/2) - cos((3 x)/2))/(2) + sin(x/2) sin(2 x)

(cos(x/2) - cos((3 x)/2))/(2) = 1/2 cos(x/2) - 1/2 cos((3 x)/2):
(cos(x/2) - cos((5 x)/2))/(2) = ^?(cos(x/2))/(2) - (cos((3 x)/2))/(2) + sin(x/2) sin(2 x)

sin(x/2) sin(2 x) = 1/2 (cos(x/2 - 2 x) - cos(x/2 + 2 x)) = 1/2 (cos(-(3 x)/2) - cos((5 x)/2)):
(cos(x/2) - cos((5 x)/2))/(2) = ^?(cos(x/2))/(2) - (cos((3 x)/2))/(2) + (cos(-(3 x)/2) - cos((5 x)/2))/(2)

Use the identity cos(-(3 x)/2) = cos((3 x)/2):
(cos(x/2) - cos((5 x)/2))/(2) = ^?(cos(x/2))/(2) - (cos((3 x)/2))/(2) + (cos((3 x)/2) - cos((5 x)/2))/(2)

(cos((3 x)/2) - cos((5 x)/2))/(2) = 1/2 cos((3 x)/2) - 1/2 cos((5 x)/2):
(cos(x/2) - cos((5 x)/2))/(2) = ^?(cos(x/2))/(2) - (cos((3 x)/2))/(2) + (cos((3 x)/2))/(2) - (cos((5 x)/2))/(2)

(cos(x/2))/(2) - (cos((3 x)/2))/(2) + (cos((3 x)/2))/(2) - (cos((5 x)/2))/(2) = 1/2 cos(x/2) - 1/2 cos((5 x)/2):
(cos(x/2) - cos((5 x)/2))/(2) = ^?(cos(x/2))/(2) - (cos((5 x)/2))/(2)

1/2 (cos(x/2) - cos((5 x)/2)) = 1/2 cos(x/2) - 1/2 cos((5 x)/2):
1/2 cos(x/2) - 1/2 cos((5 x)/2) = ^?1/2 cos(x/2) - 1/2 cos((5 x)/2)

The left hand side and right hand side are identical:
Answer: | (identity has been verified)

Jul 28, 2017
 #1
avatar+26400 
+2

Right \(\triangle{ABC}\) has AB = 3, BC = 4, and AC = 5.
Square XYZW is inscribed in triangle ABC with X and Y on \(\overline{AC}\),
W on \(\overline{AB}\), and
Z on \(\overline{BC}\).
What is the side length of the square?

 

Let s is the side length oft the square \( = \overline{XY} = \overline{YZ} = \overline{ZW} = \overline{WX}\)

Let h = \(\overline{BT}\)

Let A the area of \(\triangle{ABC}\)

 

 

h = ?

\(\begin{array}{|rcll|} \hline A &=& \frac{\overline{AB} \cdot \overline{BC} }{2} \\ A &=& \frac{3\cdot 4}{2} \\ \mathbf{A} &\mathbf{=}& \mathbf{6} \\\\ A &=& \frac{\overline{AC}\cdot h}{2} \\ A &=& \frac{5\cdot h}{2} \quad & | \quad \mathbf{A=6} \\ 6 &=& \frac{5\cdot h}{2} \\ \mathbf{h} &\mathbf{=}& \mathbf{ \frac{12}{5} } \\ \hline \end{array}\)

 

\(\mathbf{\overline{BW} =\ ?}\)

\(\begin{array}{|rcll|} \hline \frac{ \overline{BW} } {s} &=& \frac{ \overline{AB} } { \overline{AC} } \\ \frac{ \overline{BW} } {s} &=& \frac{ 3 } { 5 } \\ \mathbf{ \overline{BW} } & \mathbf{=} & \mathbf{ \frac{3}{5}s } \\ \hline \end{array}\)

 

\(\mathbf{\overline{BZ} =\ ?} \)

\(\begin{array}{|rcll|} \hline \frac{ \overline{BZ} } {s} &=& \frac{ \overline{BC} } { \overline{AC} } \\ \frac{ \overline{BZ} } {s} &=& \frac{ 4 } { 5 } \\ \mathbf{\overline{BZ}} &\mathbf{=}& \mathbf{\frac{4}{5}s } \\ \hline \end{array}\)

 

s = ?

\(\begin{array}{|rcll|} \hline A_{\triangle{ZBW}} = \frac{ \overline{BW}\cdot \overline{BZ} } {2} &=& \frac{s\cdot(h-s)} {2} \\ \overline{BW}\cdot \overline{BZ} &=& s\cdot(h-s) \quad & | \quad \mathbf{ \overline{BW} =\frac{3}{5}s } \quad \mathbf{ \overline{BZ} =\frac{4}{5}s } \quad \mathbf{h=\frac{12}{5}} \\ \frac{3}{5}s \cdot \frac{4}{5}s &=& s\cdot(\frac{12}{5}-s) \\ \frac{12}{25}s &=& \frac{12}{5}-s \\ s+\frac{12}{25}s &=& \frac{12}{5} \\ s \cdot \left(1+\frac{12}{25} \right) &=& \frac{12}{5} \\ s \cdot \left(\frac{25+12}{25} \right) &=& \frac{12}{5} \\ s \cdot \left(\frac{37}{25} \right) &=& \frac{12}{5} \\ s &=& \frac{25}{37} \cdot \frac{12}{5} \\ s &=& \frac{5}{37} \cdot 12 \\ s &=& \frac{60}{37} \\ \mathbf{s} &\mathbf{=}& \mathbf{1.\overline{621}} \\ \hline \end{array}\)

 

The side length oft the square is \(\mathbf{1.\overline{621}}\)

 

 

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Jul 28, 2017
Jul 27, 2017

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