Butuh bantuan nya dong​

gampang kk..

a + 2a + 3a + …+100a = 50

100/2(2.a + 99.a) = 50

50(101a) = 50

101a = 1

a = 1/101

sigma batas bawah k = 21, batas atas = 100

(1/101) + 5 + (2/101) + 5+ (3/101) +5 …+(100/101) +5 =

506/101 + 507/101 + 508/101 +…605/101

b =

S100 =

100/2 (2.(506/101) + 99.(1/101)) =

50(1012/101 + 99/101) =

50(1111/101) =

50(11) =

550

sigma batas bawah k = 60, batas atas = 79

(81/101)-5 + (82/101)-5 +…(90/101)-5 =

(-424/101) + (-425/101) +…(-415/101)=..

b = (-425/101) + (424/101) = -1/101

S20 = 20/2 (2.(-424/101) + 19(-1/101)

S20 = 10(-848/101 – 19/101)

S20 = 10(-867/101)

S20 = -8670/101

jadi

550 + (-8670/101) =

(5550-8670)/101 =

46880/101 =

464,…

gak ada di opsi

cari yg mendekati aja 350…