
Cosine Rule
I need to calculate the permeter of a triangle ABC
Side AB 9cm, Side BC 15cm and Angle B 110°
Using the Cosine Rule I Know
b² = a²+c²2accos B
b² = 15²+9²2x15x9xcos110°
It is claimed that
b²=398.345
b=20cm approx, a scaled drawing proves this to be the case.
My problem is, I want to make it
b² = 15²+9²2x15x9xcos110°
b² = 225+812x15x9x.3420
b² = 225+8192.34
b² = 30692.34
b² = 213.66
b = 14.62 approx which is wrong!!
If I add 92.34 to 306 I get the correct answer but I do not know where I have gone wrong. Can anybody help me?
berniecole

Hello, berniecole!
It looks like you aren't using your calculator well . . .
is in Quadrant 2, where cosine is negative.