I need help to solve this please help me...

 see proof below

Step-by-step explanation:

Use the following identities & Unit Circle calculations:

 4cos³x = 3cos x + cos 3x

--> cos³x = (1/4)[3cos x + cos 3x]

 4sin³x = 3sin x - sin 3x

--> sin³x = (1/4)[3sin x - sin 3x]

cos 60° = sin 30° = (1/2)

Proof: LHS → RHS

Given:                          cos³ 20° + sin³ 10°

Triple Angle Identity: (1/4)[3cos 20° + cos 3·20°] + (1/4)[3sin 10° - sin 3·10°]

Simplify:                      (1/4)[3cos 20° + cos 60° + 3sin 10° - sin 30°]

                                   (1/4)[3cos 20° + (1/2) + 3sin 10° - (1/2)]

                                   (1/4)[3cos 20° + 3sin 10°]

Factor:                         (3/4)[cos 20° + sin 10°]        

Proven:  (3/4)[cos 20° + sin 10°]  = (3/4)[cos 20° + sin 10°]    

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step-by-step explanation: