Given:
[tex]\alpha, \beta, \gamma[/tex] are three interior angles of any triangle.
To prove:
[tex]\cos (\alpha+\beta+\gamma)=-1[/tex]
Solution:
Angle sum property of a triangle: According to this theorem the sum of all three interior angle of a triangle is always 180 degrees.
It is given that, [tex]\alpha, \beta, \gamma[/tex] are three interior angles of any triangle.
By using the angle sum property of triangles, we get
[tex]\alpha+\beta+\gamma =180^\circ[/tex] ...(i)
We need to prove,
[tex]\cos (\alpha+\beta+\gamma)=-1[/tex]
Taking LHS, we get
[tex]LHS=\cos (\alpha+\beta+\gamma)[/tex]
[tex]LHS=\cos (180^\circ)[/tex] [Using (i)]
[tex]LHS=-1[/tex]
[tex]LHS=RHS[/tex]
Hence proved.
