Question

Resolver a P.A (-6;--9;-12;...),calcule:.
A)= a15
B)=s20
C)=s27

Answer 1

Resolução!


r = a2 - a1

r = - 9 - ( - 6 )

r = - 3



A )

a15 = a1 + 14r

a15 = - 6 + 14 * ( - 3 )

a15 = - 6 + ( - 42 )

a15 = - 48

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B )

an = a1 + ( n - 1 ) r

an = - 6 + ( 20 - 1 ) - 3

an = - 6 + 19 * ( - 3 )

an = - 6 + ( - 57 )

an = - 63



Sn = ( a1 + an ) n / 2

Sn = ( - 6 + ( - 63 ) ) 20 / 2

Sn = - 69 * 20 / 2

Sn = - 1380 / 2

Sn = - 690


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C )


an = a1 + ( n - 1 ) r

an = - 6 + ( 27 - 1 ) - 3

an = - 6 + 26 * ( - 3 )

an = - 6 + ( - 78 )

an = - 84



Sn = ( a1 + an ) n / 2

Sn = ( - 6 + ( - 84 ) ) 27 / 2

Sn = - 90 * 27 / 2

Sn = - 2430 / 2

Sn = - 1215