Question

Решите АП, плиз помогите

Answer 1

1) a₁ = - 3       a₂ = - 5

a₂ = a₁ + d

d = a₂ - a₁ = - 5 - ( - 3) = - 5 + 3 = - 2

a₁₅ = a₁ + 14d = - 3 + 14 * (- 2) = - 3 - 28 = - 31

$S_{20} =\frac{2a_{1}+19d }{2} *20=(2a_{1}+19d)*10=(2*(-3)+19*(-2))*10=(-6-38)*10=-44*10=-440$

2)  a₆ = 7                   a₉ = 10

    a₆ = a₁ + 5d           a₉ = a₁ + 8d

$-\left \{ {{a_{1}+8d=10 } \atop {a_{1}+5d=7 }} \right.\\ -------\\3d=3\\\\d=1\\\\a_{1}=7-5d=7-5*1=2\\\\a_{39}=a_{1}+38d=2+38*1=40\\\\a_{40}=a_{39}+d=40+1=41\\\\S_{40}=\frac{a_{1}+a_{40}}{2}*40=(7+41)*20=48*20=960$