4^(2x+1,5)=4^(2x)*4^(1,5)=(4^(x))^2**sqrt(4^3)=8*([green]4^(x)[/green])^2
20^(x)=(4*5)^(x)=[green]4^(x)[/green]*[blue]5^(x)[/blue]
Неравенство принимает вид:
3*5*([blue]5^(x)[/blue])^2+8*([green]4^(x)[/green])^2-22*([green]4^(x)[/green])*([blue]5^(x)[/blue]) ≤ 0
Делим на (4^(x))^2 > 0
15t^2-22t+8 ≤ 0; t=(5/4)^(x)
D=(-22)^2-4*15*8=484-480=4
t_(1)=(22-2)/30=2/3 или t_(2)=(22+2)/30=24/30=4/5
2/3 ≤ t ≤ 4/5
(2/3) ≤ (5/4)^(x) ≤ 4/5
(5/4)^(log_(5/4)(2/3)) ≤ (5/4)^(x) ≤ (5/4)^(-1)
log_(5/4)(2/3) ≤ x ≤ -1
О т в е т.[ log_(5/4)(2/3);-1]