An envelope with 96 bookmarks in it has a mass of 1467 grams. the same envelope with 49 bookmarks in it has a mass of 762 grams. find the mass of the envelope when it is empty.

Which sum or difference identity would you use to verify that cos (180° - q) = -cos q? a.) sin (a -b) = sin a cos b – cos a sin b b.) cos (a -b) = cos a cos b – sin a sin b c.) cos (a -b) = cos a cosb + sin a sin b d.) sin (a + b) = sin a cos b + cos a sin b