1)sin22,5=√(1-cos45)/2=
√(1-√2/2/2)=√(2-√2)/4=1/2*√(2-√2)
sin²75=(1-cos150)/2=(1-cos(180-30))/2=
(1+sin30)/2=(1+1/2)*1/2=3/4
OTBET 1/2*(2-√2)-3/4=1-√2/2-3/4=
1/4-√2/2
2)2cos²4x-1=0
cos8x=0
8x=π/2+πk
x=π/16+πk/8
ОДЗ
5x+1≥0⇒5x≥-1⇒x≥-0,2
Возведем в квадрат обе части
5x+1≥121
5x≥120
x≥24
x∈[24;∞)
А5=а1+4d a7=a1+6d a1+4d+a1+6d=2a1+10d=2a1+10*4=2a1+40=10
2a1+40=10 2a1=10-40 2a1=-30 a1=-30:2 a1=-15
a6=a1+5d=-15+5*4=-15+20=5