Ответ:   -16.     -1190.

Пошаговое объяснение:

Find the fifteenth term and the sum of the first 40 terms of the arithmetic progression (an) if a1=19 & d=-2.5.

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a(n) = a1+(n-1)d.

S(n)=n(a1+an)/2.

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a(15) = a1+14d=19+14(-2.5) = -16.

S(40)=40(a1+a40)/2 =>  

a(40) = 19+39*(-2.5) =-78.5 =>

S(40)=40(19+(-78.5))/2 = -1190.