Te bewijzen : | (r.cis θ)n = rn. cis nθ (DE MOIVRE) |
m.a.w. | [r.(cosθ + i.sinθ)]n = rn.(cos nθ + i.sin nθ) |
Bewijs : | r.c is θ staat voor r.(cos θ + i . sin θ) |
Deel I |
Voor de kleinste n-waarde, nl. 1 is LL = (r.cis θ)1 = r.cis θ RL = r1. cis (1.θ) = r.cis θ LL = RL → O.K. |
Deel II | Gegeven : | (r.cis θ)k = rk. cis kθ ( I.H.) |
Te bewijzen: | (r.cis θ)k+1 = rk+1. cis [(k+1)θ] | |
Bewijs : | LL = (r.cis θ)k+1 = (r.cis θ) (r.cis θ)k | |
__ = r. cis θ . rk. cis kθ | ||
__ = rk+1[(cos θ + i.sin θ)(cos kθ + i sin kθ)] | ||
__ = rk+1[cos θ.cos kθ − sin θ.sin kθ + i.(sin θ.cos kθ + cos θ.sin kθ)] | ||
__ = rk+1[cos (θ + kθ) + i.sin (θ + kθ)] | ||
__ = rk+1[cos (1+k)θ + i.sin (1+k)θ] | ||
__ = rk+1. cis [(k+1)θ] = RL Q.E.D. |