<span> (an) арифметическая прогрессия;
1) a1 =11; d =3 найдите a11.
a</span>₁₁ = a₁ + 10d = 11 + 10*3 = 41<span>
2) a1=137; d=-7 найдите S10.
S</span>₁₀ = (2a₁ + 9d)*10/2 = ( 2*137 +9*(-7))*5 = 1055<span>
3) a13=-27, a15=-13 найдите a14
a</span>₁₄ = (a₁₃ + a₁₅)/2 = (-27 - 13)/2 = -20<span>
4) a43=-208, d=-7 найдите a1
a</span>₄₃ = a₁ + 42d
<span>-208 = a</span>₁ +42*(-7)
<span>-208 = a</span>₁ - 294
<span>a</span>₁ = -208 + 294= 86<span>
5) a1=35 , a15=-21 найдите d
a</span>₁₅ = a₁ + 14d
-21 = 35 + 14d
14d = -56
d = - 4
a2=a1+d. d=a2-a1=8-(-17)=25
a7=a1+6d=-17+6×25=-17+150=133
2х-3х=-2-3
-х=-5
х=5 )))))))))