a)
cos a= -5/13
pi/2 < a < pi
sin a = √ 1-cos^2 = √ 1-(-5/13)^2 = 12/13
tg a = sin /cos = -5/13 / 12/13 = -5/12
ctg a = 1 / tg a = 1/ -5/12 = - 2,4
a)
sin a = -0.8
3pi/2 < a < 2pi
cos a = √ 1-sin^2 = √ 1- (-0.8)^2 = 0.6
tg a = sin /cos = -0.8/0.6 = - 4/3
ctg a = 1 / tg a = 1/ - 4/3 = -3/4
b)
sin a = √3/3
pi/2 < a < pi
cos a = √ 1-sin^2 = √ 1- (√3/3)^2 = -√(2/3)
tg a = sin /cos = √3/3 / -√(2/3) = -1/√2 = -√2/2
ctg a = 1 / tg a = 1 / -1/√2 = -√2
b)
cos a= 1/7
0 < a < pi/2
sin a = √ 1-cos^2 = √ 1- (1/7)^2 = 4√3/7
tg a = sin /cos = 4√3/7 / 1/7 = 4√3
ctg a = 1 / tg a = 1/ 4√3 = √3/12