What does the photography theorem mean?

in △ABC, let the opposite sides of ∠A, ∠B and ∠C be a, b and c respectively, and then there are: a=bcosC+ccosB, b=ccosA+acosC, and c=acosB+bcosA. These three formulas are called projective.

it is known that the middle angle of a triangle is A=9 degrees. AD is high.

proof 1: if the projection of point a on the straight line BC is point d, then the projections of AB and AC on the straight line BC are BD and CD, respectively, and BD=c·cosB, CD = b cosc, ∴ a = BD+CD = b cosc+c cosb can prove the rest in the same way.

proof 2: from sine theorem, we can get: b=asinB/sinA, c = asinc/Sina = asin (a+b)/Sina = a (sinacosb+cosasinb)/Sina = acosb+(asinb/Sina) COSA = a.cosb+b.