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单词 ProofOfAngleSumIdentities
释义

proof of angle sum identities


We will derive the angle sum identities for the various trigonometricfunctionsDlmfMathworldPlanetmath here. We begin by deriving the identity for the sine by meansof a geometric argument and then obtain the remaining identities byalgebraic manipulation.

Theorem 1.
sin(x+y)=sin(x)cos(y)+cos(x)sin(y)
Proof.

Let us make the restrictions 0<x<90 and0<y<90 for the time being. Then we may draw atriangle ABC such that CAB=x and ABF=y:

 ????????????wwwwwwwwwwwwwwwwABC

Since the angles of a triangle add up to 180, we must haveBCA=180-x-y, so we have sin(BCA)=sin(180-x-y)=sin(x+y).

We now draw perpendicularsPlanetmathPlanetmathPlanetmath two different ways in order to derive ratios.First, we drop a perpendicular AD from C to AB:

 ????????????wwwwwwwwwwwwwwwwABCD

Since ACD and BCD are right triangles we have, by definition,

cot(CAB)=AD¯/CD¯  cot(ABC)=BD¯/CD¯  sin(CAB)=CD¯/AC¯.

Second, we draw a perpendicular AE form A to BC. Depending on whetherx+y<90 or x+y<90 the point E will or will not lie betweenB and C, as illustrated below. (There is also the case x+y=90,but it is trivial.)

 ????????????????ΥΥΥΥΥΥΥ¨¨¨¨¨¨¨¨¨ABCE
 ??????????????¨¨¨¨¨¨¨wwwwwwwwwwwwwwwwABCE

Either way, ABE and ACE are right triangles, and we have, by definition,

sin(BCA)=AE¯/AC¯  sin(ABC)=AE¯/AB¯.

Combining these ratios, we find that

sin(BCA)/sin(ABC)=AB¯/AC¯.

To finish deriving the sum identity, we manipulate the ratios derived abovealgebraically and use the fact that AD¯+BD¯=AB¯:

sin(x+y)=sin(BCA)=AB¯sin(ABC)/AC¯
=(AD¯+BD¯)sin(ABC)/AC¯
=CD¯(cot(CAB)+cot(ABC))/sin(ABC)AC¯
=sin(CAB)sin(ABC)(cos(CAB)sin(CAB)+cos(ABC)sin(ABC))
=sin(CAB)cos(ABC)+cos(CAB)sin(ABC)
=sin(x)cos(y)+cos(x)sin(y)

To lift the restriction on the range of x and y, we use the identitiesfor complements and negatives of angles.

Entry under construction

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更新时间:2025/5/4 16:00:00