Arun
Last Activity: 7 Years ago
Dear Student
Use the formula
sin(a + b) = sina cosb + sinb cos a
Then sin(a + b + c)
= sin((a + b) + c)
= sin(a + b) cosc + cos(a + b) sin c
Then use the formula
cos(a + b) = cosa cosb - sina sinb to get
sin(a + b) cosc + cos(a + b) sin c
= (sina cosb + sinb cosa) cosc + (cosa cosb - sina sinb) sinc
= sina cosb cosc + sinb cosa cosc + sinc cosa cosb - sina sinb sinc
Regards
Arun (askIITians forum expert)
