(с-15)=729:81=9
с-15=9
с=24
X+y-8=0
x=0,y=1
0+1-8=0
-7=0
1)12*2=24
2)42-24=18
3)18:2=9
(проверка)
12+9*2=42
4 / (√5-1) + 11/(4+√5) = [ 4(4 +√5) + 11(√5-1) ] / [ (√5-1)(4+√5) ] =
= ( 16 +4√5 + 11√5 - 11) / (4√5 + (√5)² - 4 - 1√5) =
= (5+15√5) / ( 1+3√5 ) = [ 5(1+3√5) ] / (1+3√5) = 5