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### Cosines and Sines

```
Date: 6/16/96 at 14:57:1
From: Anonymous
Subject: Cosines and Sines

unbelievable to me:

cos angle A = sin (90 degrees - angle A)
```

```
Date: 6/17/96 at 12:4:38
From: Doctor Dave
Subject: Re: Cosines and Sines

Dear Boschem,

Recall that Sin(a+b) = Sin(a)Cos(b) + Cos(a)Sin(b)

and Sin(a-b) = Sin(a)Cos(b) - Cos(a)Sin(b)

also Sin(90)=1 and Cos(90)=0

so cos(a) = sin(90-a) = sin(90)cos(a) -  sin(a)cos(90)
= 1cos(a) - 0sin(a)
= cos(a)

-Doctor Dave,  The Math Forum
Check out our web site!
```
Associated Topics:
High School Trigonometry

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