Friday

March 6, 2015

March 6, 2015

Posted by **mathstudent** on Friday, December 15, 2006 at 4:19pm.

How do I get the formulas:

b * cos C + c * cos B = a

c * cos A + a * cos C = b

a * cos B + b * cos A = c

Are these standard trig formulas? What are they called? Or where are they derived from?

thanks!

b * cos C + c * cos B = a

Draw the line at right angles to side a that splits the angle A (not necessarily in equal parts). Then, the triangle is slit in two right triangles, and you see that

a = a1 + a2

And:

a1 = b * cos C

a2 = c * cos B

Cyclically permuting a -- > b---> c ---> a (and the uppercase variables) give you the other two equations)

**Answer this Question**

**Related Questions**

geometry - If a triangle has sides of lengths a and b, which make a C-degree ...

Geometry - x ________________<20* |<90* y | |<70* (hypotenuse length ...

SAT math - Can someone please double check my true and false answers! 1. All ...

Pre-Calc - Consider a triangle ABC. C is a right angle, and side a is opposite ...

math - A right triangle has a hypotenuse of length 3.00 m, and one of its angles...

maths - shapes - Can you name these quadrilaterals? a) All sides are the same ...

Math - Yesterday I learned to solve a right triangle assuming I know the 3 ...

geomerty - The following triangles, Ä ABC and Ä DEF, are congruent. abc 67....

math - If two angles of a triangle are unequal, the sides opposite these angles ...

geometry - An angle bisector of a triangle divides the opposite side of the ...