Question

Prove that:
sin^3A+ sin^3(120+A) +sin^3(240+A)= -3/4 sin3A

Answer 1

Sorry for the late reply:

The important thing to note in this question is the simplification part. Expanding sin^3(120+a) and sin^3(240+a) is the worst option. Instead make a simple transformation and you get it in 2 steps.

Use the formula sin 3A=3sin A-4sin^3 A
From this: sin^3A=(sin3A-3sin A)/-4 -----------(1)

Replace all the sin^3 terms in qn by (1) and simply add and you are done!

Let me know if you are not still getting it! Good luck!