a. sin100o +sin80o + cos16o + cos164o
= sin100o +sin100o + cos16o- cos16o
= 2sin100o
b. 2sin(180o−α).cotα – cos(180o−α)tanα .cot(180o−α)
= 2sinα.cotα + cos(α)tanα .(-cot(α)
= 2cosα - cosα
= cosα