2. если а6-а4=-4, то d=-2
S3=(а1+аn)/2 * n , 30=(a1-10)/2*3 , 30=(3a1-30)/2 , 60=3a1-30 , 3a1=90 , a1=30
an=a1+(n-1)d , -10=30+(n-1)*(-2) , -10=30-2n+2 , 2n=2+30+10 , 2n=42 ,
n=21
1. Sn=(a1+an)/2 * n , 22=(a1+a11)/2 * 11 , 22=(11a1+11a11)/2 , 44=11a1+11a11 , 4=a1+a11 .
В эту формулу подставляем наши данные :
получаем а4+а8=а1+а11 б а4+а8=4
и 2а6=а1+а11 , 2а6=4
<span>а4+2а6+а8 - ? , (а4+а8)+2а6=4+4=8</span>
Пусть х км в час - скорость течения реки,
(12+х) км в час - скорость по течению
(12-х) км в час - скорость против течения
39/х ч- время по течению
21/(12-х) - время против течения
Они равны.
Уравнение:
39/(12+х)=21/(12-х)
Пропорция.
21·(12+х)=39·(12-х)
21·12+21х=39·12-39х
21х+39х=39·12-21·12
60х=12·(39-21)
60х=12·18
х=3,6
О т в е т. 3,6 км в час - скорость течения реки
4x+16y=4(x+4y)
3a²+a²=4a²
Решите уравнение
3x²+7x=0
х(3х+7)=0
х=0 или 3х+7=0
3х=-7
х=-7/3
Разложите на множители
2xyz-3x²z²+4y²z
=z(2xy-3x²z+4y²)
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