90 000 - 705×83 = 31 485
80 100 - 603×79 = 32 463
3(1-cos^2x) -10cosx-6=0
3-3cos^2x-10cos-6=0
-3cos^2x-10cos-3=0
3cos^2x+10cos+3=0
а дальше уже по дискриминанту
<span>1) 5/8 - 2/5 + 3/20=25-15-6/40=15/40=3/8
</span><span>2) 1/8 + ( 5/9 -1/4 )=
1)5/9-1/4=20-9/36=11/36
2)11/36+1/8=22+9/72=31/72
</span><span>3) 5/6 - ( 3/10 + 2/5 )=
</span>1)3/10+2/5=7/10
2)5/6-7/10=25-21/30=4/30=2/15
<span>4) 3/4 + 1/8 - 5/16=12+2-5/16=9/16</span>
Х - девочек
у - мальчиков
{ x+y=21
{ 2x+3y=54⇔
{ x=21-y
{ 2(21-y)+3y=54⇔
{ x=21-y
{ 42-2y+3y=54⇔
{ x=21-y
{y=12 (мальчиков)
21-12=9(девочек)