Trignometry

prnt.sc/jnp0iz
PLEASE HELP ME!!!!!

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  1. Derive this identity from the sum and difference formulas for cosine:
    sinasinb=1/2[cos(a-b)cos(a+b)]
    Start with the right-hand side since it is more complex.
    Calculations:
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    Reasons:
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  2. we know:
    cos(a-b) = cosa cosb + sina sinb , and
    cos(a+b) = cosa cosb - sina sinb
    ----------------------------------------- , let's subtract them
    cos(a-b) - cos(a+b) = 2sina sinb
    (1/2)( cos(a-b) + cos(a+b) = sina sinb , as required

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