How do I simplify $\sqrt{3}(\cot 70^\circ + 4 \cos 70^\circ )$? [duplicate]

Solution 1:

$$\cot(70)+4\cos(70) = \frac{\cos(70)}{\sin(70)} + 4\cos(70) =\frac{\cos(70)+4\cos(70)\sin(70)}{\sin(70)} = \frac{\cos(70)+2\sin(140)}{\sin(70)} = \frac{(\cos(70)+\sin(140))+\sin(140)}{\sin(70)} = \frac{(\sin(20)+\sin(140))+\sin(140)}{\sin(70)} = \frac{2\sin(80)\cos(60)+\sin(140)}{\sin(70)} = \frac{2\sin(110)\cos(30)}{\sin(70)} = 2 * \frac{\sqrt3}{2} = \sqrt 3$$

Note $\sin(110)=\sin(70)$ at the end. $\cos(60)=\frac{1}{2}$ and $\cos(30)=\frac{\sqrt3}{2}$

Identities used: $\sin(90-x)=\cos(x)$

$\sin(a+b)=\sin(a)\cos(b)+\cos(a)\sin(b)$

$\sin(2x)=2\sin(x)\cos(x)$

$\sin(a)+\sin(b)=2\sin(1/2(a+b))\cos(1/2(a-b))$

Solution 2:

To continue from your method:

$$\sqrt{3}\frac{(sin20+2sin40)}{cos20}=2\left(\frac{\frac{\sqrt{3}}{2}sin20+2\frac{\sqrt{3}}{2}sin40}{cos20}\right)$$ Using $\frac{\sqrt{3}}{2}=cos30$ we get

$$2\left(\frac{\frac{\sqrt{3}}{2}sin20+2\frac{\sqrt{3}}{2}sin40}{cos20}\right)=\left(\frac{2cos30sin20+2 \times 2cos30sin40}{cos20}\right)=\frac{sin50-sin10+2(sin70+sin10)}{cos20}=\frac{sin50+sin10+2sin70}{cos20} $$

But $sin50+sin10=cos20$ and $sin70=cos20$ so

$$\frac{sin50+sin10+2sin70}{cos20}=\frac{cos20+2cos20}{cos20}=3 $$