Question

if A+B+C=180 then prove cos(A+B)=-cosC​

Answer 1

Step-by-step explanation:

Given:

A + B + C =180

B = 180 - C - A

Substituting in cos(A+B)

=cos(A+180-C-A)

=cos(180-C)

=cos(π-C)

= -cosC { cos(π-thetha)= -cos(thetha) }

HENCE PROVED

HOPE IT HELPS.

Answer 2

Given:

A+B+C=180

=>A+B=180-C

Answer:

Since, A+B=180-C,

Putting Cos on both sides, we get :

cos(A+B) = cos(180-C)

And,

cos(180-C)= cos(π-C)

cos(π-C)= - cosC

Hence, proved.