The method consists of examining every possible case of the hypothesis It is
practically convenient only when the number of possible cases are few Example 4 Show that in any triangle ABC,
a = b cos C + c cos B
Solution Let p be the statement “ABC is any triangle” and q be the statement
“a = b cos C + c cos B”
Let ABC be a triangle From A draw AD a perpendicular to BC (BC produced if
necessary)