a) Vì α là góc tù nên cos α < 0

Ta có: sin²α + cos²α = 1

Suy ra: cos²α = 1 – sin²α = 1−19=89

cos α = ±89=±223

Vì 90° < α < 180° nên cos α = −223

tan α = sinαcosα=13:−223=−122

cot α =  1tanα=−22

b) Ta có:

cot(180°– α) = – cotα

cos(180°– α) = – cosα

cot(90°– α) = tanα

Khi đó: A = sinα . cot (180° – α) + cos(180° – α). cot (90°  – α)

= sinα . (–cotα) + (–cosα). tanα = 13.22+22 3.−122=223−13=22−13

B = 3sinα+2cosα−2sinα−2cosα

= 3.13+2.−223−213−2.−223=1−4−213+43=−3.