Question Number 9218 by nazar last updated on 23/Nov/16
$$\mathrm{If}\:\mathrm{sin}\:{A}+\mathrm{sin}\:{B}={a}\:\mathrm{and}\:\mathrm{cos}\:{A}+\mathrm{cos}\:{B}={b}, \\ $$$$\mathrm{then}\:\mathrm{cos}\:\left({A}+{B}\right)= \\ $$
Commented by sandy_suhendra last updated on 24/Nov/16
$$\mathrm{cos}\left(\mathrm{A}+\mathrm{B}\right)\:\mathrm{or}\:\mathrm{cos}\left(\mathrm{A}−\mathrm{B}\right)\:? \\ $$