Inverse Trig

[unknown id]

Got stuck on this question. Any help would be appreciated:

Prove that

[Mao]

maybe try taking cos on both sides then expanding the RHS with compound angle formula???

[unknown id]

i could have done that but i didn't want to touch the right-hand side when proving

[Mao]

why not?









QED

you can also use the symmetry of cos:





we also know that







QED

i think this would be your preferred method?

[unknown id]

i thought that whenever we have to prove that the left side equals the right side, only the left side should be used in our calculations

[Mao]

i thought that whenever we have to prove that the left side equals the right side, only the left side should be used in our calculations

however (more realistically), if you can arrive at 0=0 by any means, it'll be eureka! regardless

i have since edited my post to include a second proof with your preferred "only involve the left side"
even though the beginnning step is not

[unknown id]

thanks mao, the second method does seem more preferable

[gfb]

Good work Mao .