The matrix below represents a system of equations ( 2 -1 -3 -20 -2 -6 6 -4 0 3 -2 -9). Which matrix represents 1/2R2+R3-R3 ?

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