Помогите решить задания по алгебре

7)
{ log(y) x - log(x) y = 8\3 ------> log(y\x) - 1\log(y) x = 8\3
{ xy = 16
=>
3*(log(y)x)^2 - 8log(y) x - 3 = 0 ----> log(y) x = t
3t^2 - 8t - 3 = 0 ------> t1 = -1\3; t2 = 3 =>
log(y) x = t1 = - 1\3 -----> x1 = y^(-1\3) = 1\y^3
log(y) x = t2 = 3 ----------> x2 = y^3
=>
xy = 16
x1 = 1\y^3 ----> 1\y^3 * y = 16 -----> 1\y^2 = 16 ----> y(1,2) = + -1\4
x2 = y^3 -------> y^3 * y = 16 ------> y^4 = 2^4 -----> y(1,2) = + -2
8)
{ x - y = 90
{ lg x + lg y = 3 -------> ОДЗ: x>0; y>0
=>
{ x - y = 90 --------> y = x - 90
{ lg xy = 3 ---------> xy = 10^3 = 1000
=>
x * (x - 90) = 1000 ------> x^2 - 90x - 1000 = 0
Дальше легко
9)
{ log2 x + log2 y = 2 -----> ОДЗ: x>0; y>0
{ log3 (y - x) = 1 -----------> ОДЗ: y>x
=>
{ xy = 2^2
{ y - x = 3^1
Дальше легко