Question

uuuu🦖

求解 謝謝！🥹🙏🏻

)10. 已知一等差數列a, a2,a3,………, a,若 Sp=600,且a2+az+……+aso=2400,
S50 = ?
(A) 2480
(B) 2500
(C) 2560
(D) 2600
5口(0)
ORE ALAJI 2 DR (0)

Answer 1

✨ คำตอบที่ดีที่สุด ✨

kevin_maphy

a21=a1+(21-1)d=a1+20d
a22=a2+(22-2)d=a2+20d
a23=a3+(23-3)d=a3+20d
...
a50=a30+(50-30)d=a30+20d
a21+a22+...+a50
=(a1+20d)+(a2+20d)+...+(a30+20d)
=a1+a2+...+a30+30*20d
=600+600d
600+600d=2400
d=3

s30=(30/2)*(2a1+(30-1)*3)
=30a1+1305
30a1+1305=600
a1=-47/2

s50=(50/2)*(2*(-47/2)+(50-1)*3)
=25*100
=2500

答案為(B)