Question

The value cos-1(-3/7) + cos-1(2/7) + cos-1(6/7) is ____________

(a) 215.4˚

(b) 273.4˚

(c) 188.4˚

(d) 219.4˚

I had been asked this question during an online exam.

My doubt stems from Cartesian Vectors in portion Force Vectors of Engineering Mechanics

Answer 1

Correct choice is (d) 219.4˚

Easiest explanation: It’s a calculative question, but still can be solved if you construct the triangle of the respective cosine inverses given, then convert them in the tangent form then use log table, if necessary. Or the use of a calculator is also good if permissible. Or you can make use of triangle laws to obtain the same.