Math Is Fun Forum

isodel

Member

Registered: 2008-01-04

Posts: 2

Proving identities

I really need help with this, I just can't figure this out.

1. (cosx - sinx)² = 1- 2sinxcosx

2. cosx^4 - sinx^4 = cosx² - sinx²

3. 1/secx + tanx = secx - tanx

TheDude

Member

Registered: 2007-10-23

Posts: 361

Re: Proving identities

The first 2 of these uses the rule

1. Multiply out the left side to get

2. Factor the left side to get

3. Remember that tan(x) = sin(x) / cos(x) and sec(x) = 1 / cos(x).

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