Thread: Proving Trig Identities.

Prove that cos^-1(1/5)+cos^-1(5/7)+cos^-1(19/35) = Pie without the use of a calculator? How would I do this?

Originally Posted by zweevu

Prove that cos^-1(1/5)+cos^-1(5/7)+cos^-1(19/35) = Pie without the use of a calculator? How would I do this?

Several approaches are possible. One approach might be the following:

Let , and .

Now evaluate by applying the compound angle formula several times.

Note: Find , and by using the known values for cos and applying the Pythagorean Identity.