<span>Составляем систему </span>
<span>a + b + c = a + 4 </span>
<span>a + b + c = b + 8 </span>
<span>a + b + c = c + 10 </span>
<span>a + b = 10 </span>
<span>a + c = 8 </span>
<span>b + c = 4 </span>
<span>Складываем все три уравнения </span>
<span>2a + 2b + 2c = 10 + 8 + 4 = 22 </span>
<span>a + b + c = 11 </span>
<span>c = 1, b = 3, a = 7</span>
( 2x + 7)² = ( 2x - 5)²
( 2x + 7)( 2x + 7) = ( 2x - 5)( 2x - 5)
4x² + 28x + 49 = 4x² - 20x + 25
4x² - 4x² + 28x + 20x = 25 - 49
48x = - 24
x = - 24 ÷ 48
x = - 0,5
Ответ: x = - 0,5
А - 5)
В - 3)
С - 1)
...............................................................................................................
Cos x = -sqrt(2)/2
X=+-3π/4+2πn
Без десяти три - 14 : 50 либо 2 : 50.
Удачи)