@kc_kennylau why do you subtract and then add the sin^2 t ?

Question

sammixboo · Answer

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anonymous · Answer

ok thx! @sammixboo

kc_kennylau · Answer

What was the question?

anonymous · Answer

cos^4t - sin^4t = 1-2sin^2t you answered this question asked by @queenbeedoe

kc_kennylau · Answer

$$\cos^4t-\sin^4t$$$$=(\cos^2t+\sin^2t)(\cos^2t-\sin^2t)$$$$=\cos^2t-\sin^2t$$$$=\cos^2t+\sin^2t-2\sin^2t$$$$=1-2\sin^2t$$

kc_kennylau · Answer

In order to form $\cos^2t+\sin^2t$.

anonymous · Answer

Oh i see but when you were solving for @queenbeedoe you wrote the third step like this: 
= cos^2t + (sin^2t - sin^2t) - sin^2t   so i was confused.

anonymous · Answer

OHHHHHH i understand it now!!! nevermind im so sorry im so stupid. You wrote it like that to get a "1"  for 1- 2sin^2t  am i correct?

kc_kennylau · Answer

Yep :)