Ranbu
Скорее всего в условии опечатка sin3a=3sina-4sin^3..докажем: sin3a=sin(a+2a)=sina*cos2a+sin2a*cosa sin3a=sina(1-2sin^2)+cosa(2sina*cosa) sin3a=sina-2sin^3a+2sina*cos^2a sin3a=sina-2sin^3a+2sina(1-sin^2a) sin3a=sina-2sin^3a+2sina-2sin^3a=3sina-4sin^3a(ч.т.д)
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sin3a=3sina-4sin^3..докажем:
sin3a=sin(a+2a)=sina*cos2a+sin2a*cosa
sin3a=sina(1-2sin^2)+cosa(2sina*cosa)
sin3a=sina-2sin^3a+2sina*cos^2a
sin3a=sina-2sin^3a+2sina(1-sin^2a)
sin3a=sina-2sin^3a+2sina-2sin^3a=3sina-4sin^3a(ч.т.д)