А15=a₁+(n-1)d
a15=9,1+(9,1-1)*(-2,5)
a15=-25,9
S15= (a₁+a15)*n/2=(9,1-25,9)*15/2
S15=-126
Ответ: -126.
36у^2-16=0
36у^2=16
у^2=16/36
у=±4/6=±2/3
<span>1/2log_0.4(5*x+1)=log_0.4(x-1)
</span>√(5x + 1) = x - 1
ОДЗ: x - 1 > 0, x > 1
(√(5x + 1))^2 = (x - 1^2
5x + 1 = x^2 - 2x + 1
x^2 - 7x = 0
x(x - 7) = 0
x1 = 0
x2 = 7