А) 1/3 (8,57²-5,57²)-3*8,57*5,57=
=1/3 (8,57-5,57)(8,57²+8,57*5,57+5,57²)-3*8,57*5,57=
=1/3 * 3 *(8,57²+8,57*5,57+5,57²)-3*8,57*5,57=
=8,57²+8,57*5,57-3*8,57*5,57+5,57²
=8,57²-2*8,57*5,57+5,57²
=(8,57-5,57)²=3²=9
б) 1/9 (2,76³+6,24³)+3*2,76*6,24=
=1/9 (2,76+6,24)(2,76²-2,76*6,24+6,24²)+3*2,76*6,24=
=1/9*9 (2,76²-2,76*6,24+6,24²)+3*2,76*6,24=
=2,76²-2,76*6,24+3*2,76*6,24+6,24²=
=2,76²+2*2,76*6,24+6,24²=
=(2,76+6,24)²=9²=81
Х - скорость лодки
<span>10/(х+3) + 12/(х-3) = 2 </span>
<span>10 * (х-3) + 12 * (х+3) = 2*(х-3)*(х+3) </span>
<span>10х - 30 + 12х + 36 = 2x^2 - 18 </span>
<span>2x^2 - 22x - 24 = 0 </span>
<span>x^2 - 11x - 12 = 0 </span>
<span>x1 = 12 </span>
<span>х2 = -1 (не удовлетворяет условию) </span>
<span>Ответ: 12 км/час</span>
4x^2-12x+9-11x+19=0
4x^2-23x+9+19=0
4x^2-23x+28=0