Question

calcule a soma dos 15 primeiros termos da pa (7,15,...)

Answer 1

a15 = 7 + 14*8

s15 = (7 + 119)*15/2
s15 = 63*15
s15 = 945

Answer 2

$calcular~~a_n=a_{15}=? \\ \\ a_1=7 \\ n=15 \\ r=15-7=8 \\ \\ a_n=a_1+(n-1)r \\ \\ a_{15}=7+(15-1)(8) \\ a_{15}=7+(14)(8) \\ a_{15}=7+112 \\ a_{15}=119$

$Soma \\ \\ a_n=a_{15}=119 \\ a_1=7 \\ n=15 \\ \\ S_n= \frac{(a_1+a_n)n}{2} \\ \\ S_{15}= \frac{(7+119)15}{2} \\ \\ S_{15}= \frac{(\not126^{63})(15)}{\not2} \\ \\ S_{15}=(63)(15) \\ \\ S_{15}=945$