10sinx/2cosx/2=3sin²x/2+3cos²x/2-2cos²x/2+2sin²x/2=0
5sin²x/2-10sinx/2cosx/2+cos²x/2=0/cos²x/2
5tg²x/2-10tgx/2+1=0
tgx/2=a
5a²-10a+1=0
D=100-20=80
a1=(10-4√5)/10=1-0,4√5⇒tgx/2=1-0,4√5⇒x/2=arctg(1-0,4√5)+πn⇒
x=2arctg(1-0,4√5)+2πn,n∈z
a2=1+0,4√5⇒tgx/2=1+0,4√5⇒x/2=arctg(1+0,4√5)+πn⇒
x=2arctg(1+0,4√5)+2πn,n∈z
1) a₇ + a₁₃ = 21
a₁ + 6d + a₁ + 12d = 21
2a₁ + 18d = 21
a₁ + 9d = 10,5
2) a₈ + a₁₂ - a₁₅ = 3
a₁ + 7d + a₁ + 11d - (a₁ + 14d) = 3
2a₁ + 18d - a₁ - 14d = 3
a₁ + 4d = 3
5-2х в квадрате = 25-20х+4х^=25+80+64=169