-х-4+5х+15=-5-5х-2
4х+11=-7-5х
4х+5х=-11-7
9х=-18
х=-2
A₁ = 24; a₂ = 21.
d = a₂ - a₁ = 21 - 24 = -3
S₁₀ = 10·(2a₁ + 9d)/2 = 5(2a₁ + 9d) = 10a₁ + 45d = 10·24 + 45·(-3) = 105
Ответ: S₁₀ = 105.
Abc+a2c+a2b+b2c+abc+b2a+c2a+c2b+abc-abc=(bc+ab+c2+ac)(a+b)
2abc+a2c+a2b+b2c+b2a+c2a+c2b=abc+b2c+a2b+ab2+c2a+c2b+a2c+abc
2abc+a2c+a2b+b2c+b2a+c2a+c2b-abc-b2c-a2b-ab2-c2a-c2b-a2c-abc=0
1)Sin^2a -sin^2b=(sina-sinb)(sina+sinb)==2sin(a+b)/2*cos(a-b)/2*2sin(a-b)/2cos(a+b)/2==2sin(a-b)/2cos(a-b)/2*2sin(a+b)/2cos(a+b)/2==sin(a-b)sin(a+b)2)tg(pi/4+a/2)-tg(pi/4-a/2)=sin(pi/2+a)/(1+cos(pi/2+a))- -sin(pi/2-a)/((1+cos(pi/2-a))==cosa/(1-sina)-cosa/(1+sina)=(cosa+sinacosa-cosa+sinacosa)/(1-sin^2a)==2sinacosa/cos^2a=2sina/cosa=2tgaa-альфа b-бета