Aritmetika 1.tentukan jumlah dari deret berikut.. a.12+15+18+......+99 b.33+29+25....+(-143) c.1/6+2/3+7/6+5/3+.......+73/4 d.9+16+23+30+......+702 2.dalam suat

a=20
b=4
n=18
[tex] Sn=\frac{n}{2}(2a+(n-1)*b)[/tex]
[tex]S18 =\frac{18}{2}(2*20+(18-1)*4)[/tex]
[tex]S18= 9 (40+17*4) [/tex][tex]S18= 9 (40+68)[/tex]
[tex]S18=9(108)[/tex]
[tex]S18=972[/tex]
∴ didalam aula ada 972 kursi

saya coba y

1.
A.
a = 12
b = 18 - 15 = 3
Un = 99

Un = a + (n-1)b
99 = 12 + (n - 1)3
99 = 12 + 3n - 3
3n = 99 - 12 + 3
3n = 90
n = 30

Sn = n/2 (a + Un)
S30 = 30/2 (12 + 99)
S30 = 15 ( 111)
S30 = 1.665

B.
a = 33
b = 29 - 33 = -4
Un = -143

Un = a + (n-1)b
-143 = 33 + (n - 1)(-4)
-143 = 33 - 4n + 4
-143 = 37 - 4n
4n = 37 + 143
4n = 180
n = 45

S45 = 45/2 (12 + (-143)
S45 = 22,5 (-131)
S45 = -2.947,5

C.
a = 1/6
b = 2/3 - 1/6 = 3/6
Un = 73/3

Un = a (n - 1)b
73/3 = 1/6 + (n - 1)3/6
73/3 = 1/6 + 3/6 n - 3/6
73/3 = - 2/6 + 3/6 n
3/6 n = 73/3 + 2/6
3/6 n = 148/6
n = 148/6 × 6/3
n = 148/3

S148/3 = 148/3 / 2 (12 + 73/3)
S148/3 = 148/6 ( 99/3)
S148/3 = 74/3 × 33
S148/3 = 74 × 11
S148/3 = 814

D.
a = 9
b = 16 - 9 = 7
Un = 702

Un = a + (n-1)b
702 = 9 + (n - 1)7
702 = 9 + 7n - 7
7n = 702 - 2
7n = 700
n = 100

S100 = 100/2 (12 + 702)
S100 = 50 (714)
S100 = 35.700

2.
a = 20
b = 4
S18.....

Sn = n/2 ( 2a + (n -1)b)

S18 = 18/2 (2.20 + (18-1)4)

S18 = 9 (40 + 68)

S18 = 9(108)

S18 = 972

semoga berguna +_+