View Full Version : Sine rule using cross product
Could any one tell me how to use the cross product to prove the sine rule
rl.bhat
Oct20-09, 12:01 PM
Area of a triangle of side a.b and c is
A = 1/2*axb = 1/2absinC
Similarly 1/2*bxc = 1/2 bcsinA and so on
So
absinC = bcsinA = casinB. Dividing abc to all we get
sinA/a = sinB/b = sinC/c
luisfigo777
Oct20-09, 12:11 PM
Here AB,BC,CA ,a,b,c are vectors and AB=a BC=b CA=c
in a triangle ABC,
AB + BC + CA = 0
a + b + c= 0
a x b =b x c = c x a(proved using above statement just take b and c to other side and take crossproduct with b on both sides first and then with c)
la x bl= |b x c|= |c x a |
|a||b| SinC= |b||c|SinA=|a||c| SinB
dividing by |a||b||c|
we get Sine formula
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