For a triangle $A B C$, the value of $\cos 2 A+\cos 2 B+\cos 2 C$ is least If its inradius is $3$ and incentre is $M$, then which of the following is NOT correct?
- area of $\triangle A B C$ is $\frac{27 \sqrt{3}}{2}$
- $\sin 2 A+\sin 2 B+\sin 2 C=\sin A+\sin B+\sin C$
- perimeter of $\triangle A B C$ is $18 \sqrt{3}$
- $\overrightarrow{M A} \cdot \overrightarrow{M B}=-18$
The Correct Option is A
Approach Solution - 1
If cos2A+cos2B+cos2C is minimum then A= B=C=60∘
So △ABC is equilateral
Now in-radias r=3
So in △MBD we have
Tan30∘=BDMD=a/2r=a6
1/3=a1=a=63
Perimeter of △ABC=183
Area of △ABC=43a2=273
Approach Solution -2
If \( \cos 2A + \cos 2B + \cos 2C \) is minimum, then \[ A = 60^\circ, \quad B = C = 60^\circ \] \[ \Rightarrow \triangle ABC \text{ is equilateral} \] \[ \text{In-radius} = r = 3 \]
\[ \text{Area} = \frac{\sqrt{3}}{4} a^2 = 27\sqrt{3} \] \[ \text{Thus, area is incorrectly given as } \frac{27\sqrt{3}}{2} \]
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Concepts Used:
Trigonometric Functions
The relationship between the sides and angles of a right-angle triangle is described by trigonometry functions, sometimes known as circular functions. These trigonometric functions derive the relationship between the angles and sides of a triangle. In trigonometry, there are three primary functions of sine (sin), cosine (cos), tangent (tan). The other three main functions can be derived from the primary functions as cotangent (cot), secant (sec), and cosecant (cosec).
Six Basic Trigonometric Functions:
- Sine Function: The ratio between the length of the opposite side of the triangle to the length of the hypotenuse of the triangle.
sin x = a/h
- Cosine Function: The ratio between the length of the adjacent side of the triangle to the length of the hypotenuse of the triangle.
cos x = b/h
- Tangent Function: The ratio between the length of the opposite side of the triangle to the adjacent side length.
tan x = a/b
Tan x can also be represented as sin x/cos x
- Secant Function: The reciprocal of the cosine function.
sec x = 1/cosx = h/b
- Cosecant Function: The reciprocal of the sine function.
cosec x = 1/sinx = h/a
- Cotangent Function: The reciprocal of the tangent function.
cot x = 1/tan x = b/a
Formulas of Trigonometric Functions:
