Answer :

Question
Sin α . Cos α . Tan α =  (1 – Cos α)  (1 + Cos α)

Answer
Left side  = Sin β . Tan β + Cos β 
               = Sin β . Sin β / Cos β + Cos β
               = Sin² β / Cos β + Cos² β / Cos β
               = 1 / Cos β = Sec β = Right side proven
AL2006
Let's work on the left side first. And remember that
the tangent is the same as sin/cos.

sin(a) cos(a) tan(a)

Substitute for the tangent:

[ sin(a) cos(a) ] [ sin(a)/cos(a) ]

Cancel the cos(a) from the top and bottom, and you're left with

[ sin(a) ] . . . . . [ sin(a) ] which is [ sin²(a) ]  That's the left side.

Now, work on the right side:

[ 1 - cos(a) ] [ 1 + cos(a) ]

Multiply that all out, using FOIL:

[ 1 + cos(a) - cos(a) - cos²(a) ]

= [ 1 - cos²(a) ] That's the right side.

Do you remember that for any angle, sin²(b) + cos²(b) = 1  ?
Subtract cos²(b) from each side, and you have sin²(b) = 1 - cos²(b) for any angle.

So, on the right side, you could write [ sin²(a) ] .

Now look back about 9 lines, and compare that to the result we got for the left side .

They look quite similar. In fact, they're identical. And so the identity is proven.

Whew !





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