Ариф. прогрессия
а1 = 1
n = 99
s= (а1 + an) * 99 / 2
S = 100*99/2 = 4950
3420уч.- 1/10
всего? учебников
1)3420х10= 34200 учебников было в биьлиотеке
2)3420+34200= 37620= учебников стало в библиотеке.
<span>х(х+10)=119
</span><span>х²+10х-119=0
</span>D = b² - 4ac = 10² - 4·1·(-119) = 100 + 476 = 576
x₁<span><span> =(</span><span><span>− 10 + √576)/</span>2 · 1</span>=(<span>− 10 + 24)/2</span>=<span>14/2</span>=<span>7
</span></span>x₂<span><span> =(</span><span><span>− 10 − √576)/</span>2 · 1</span>=(<span>− 10 − 24)2</span>=<span>− 34/2</span>=−<span>17
</span></span>х*(х-18)=115
<span>х²-18х-115=0
</span>D = b² − 4ac = (− 18)²<span> − 4 · 1 · (− 115) = 324 + 460 = 784
</span>x₁<span> =(</span><span><span>18 + √784)/</span>2 · 1</span>=(<span>18 + 28)/2</span>=<span>46/2</span>=<span>23
</span>x₂<span><span> =(</span><span><span>18 − √784)/</span>2 · 1</span>=(<span>18 − 28)/2</span>=<span>− 10/2</span>=−<span>5
</span></span>х(х+2)=143
<span>х²+2х-143=0
</span>D<span> = </span>b²<span> − 4</span>ac<span> = 2</span>²<span> − 4 · 1 · (− 143) = 4 + 572 = 576
</span><span><span>x</span></span>₁<span><span> =(</span><span><span>− 2 + √576)/</span>2 · 1</span>=(<span>− 2 + 24)/2</span>=<span>22/2</span>=11</span>
<span><span>x</span></span>₂<span><span> =(</span><span><span>− 2 − √576)/</span>2 · 1</span>=(<span>− 2 − 24)/2</span>=<span>− 26/2</span>=−<span>13
</span></span>
A=8см
b=8-2=6см
S=a*b=8*6=48см²
Р=2*(a+b)=2*(8+6)=28см