Sn=b1*(g^n-1)/(g-1) 80=b1*(3^4-1)/(3-1), 80=b1*40, b1=80/40=2
b3=b1*g^2=2*9=18
Габв вот ответ по возрастанию
(a-b)^2/ab+b^2:a^2-b^2/b=(a-b)^2/b(a+b)*b/(a-b)*(a+b)=a-b/(a+b)^2
1.4 sin(7π/6)+ctg(5π/4) = 4 sin(π + π/6)+ctg(π + π/4) = 4 sin(π + π/6)+ctg(π + π/4) = -4sin(π/6) + ctg(π/4) = -2 + 1 = -1
2. 2cos^2a-cos2a=2cos^2a - cos^2a + sin^2a = cos^2a+sin^2a=1
3.sin2a=2sina*cosa
cosa=-1/√5
sina=√(1-( -1/√5)^2) = √(1-1/5)= √(4/5) = 2/√5
Sin2a = 2 * 2/√5 *(-1)/√5 = -4/5 = -0,8