9+ 99/99 = 10
99/9 – 9/9 = 10.........
косинус во второй четверти отрицательный,
sin^2B=0,64
cos^2B=1-sin^2b=1-0,64=0,36
cosB=-0,6
sin(П/4+B)=sinП/4cosB+cosП/4sinB=
=√2/2(0,8-0,6)=(√2/2)*0,2=√2*0,1≈0,144
Решение
Ctg (pi/3-1/4x)=5/12
сtg(1/4x - π/3) = - 5/12
(1/4)x - π/3 = arctg(- 5/12) + πk, k ∈Z
(1/4)x = π/3 - arctg(5/12) + πk, k ∈Z
x = 4π/3 - 4arctg(5/12) + 4πk, k ∈Z