B1 = ¼; q = 1/2; bn = 1/512
Sn - ?
bn = b1 * q^(n-1) => q^(n-1) = bn/b1
q^(n-1) = 1/128
1/2^(n-1) = (1/2)^7
n - 1 = 7
n = 8
Sn = b1 * (qⁿ - 1)/(q - 1) = 1/4 * (1/256 - 1)/(1/2 - 1) = 1/4 * 2*255/256 = 255/512
Ответ: 255/512
4/(19x+1)=1/5
19x+1=20
19x=20-1
19x=19
x=1
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2х - 6х + 5 = 45
-4х= 45-5
-4х=40
х=40:(-4х)
х=-10
ответ -10