minta bantuannya dong, no 10-14 materinya blom dijelasin, tapi udah dikasih tugas duluan

No. 10
Sn=½n(2a+(n-1)b))
⅛(32n-n²) =½n(2a+bn-b)
4n-⅛n²=an+½bn²-½bn
4n-⅛n²=(a-½b)n+½bn²
½b=-⅛
b=-⅛:½
b=-⅛ x 2=-¼
a-½b=4
a-½.(-¼) =4
a+⅛=4
a=4-⅛=31/8
a. Suku pertama=31/8=3 ⅞
b. U4=a+3b=31/8+3.(-¼)
=31/8-3/4=31/8-6/8=25/8=3 ⅛
c. Beda=b=-¼
No. 11
a=-5
b=-1-(-5)=-1+5=4
Sn=130
½n((2.(-5)+(n-1)4)) =130
½n(-10+4n-4)=130
2n²-7n-130=0
2n²-20n+13n-130=0
2n(n-10)+13(n-10)=0
(2n+13)(n-10)=0
n=-13/2 atau n=10
Ambil yg +, maka n=10
No. 12
U4=13
a+3b=13 (i)
U10=31
a+9b=31 (ii)
Persamaan (i) (ii)
Eliminasi a
-6b=-18
b=3
a+3b=13
a+3.3=13
a=13-9=4
S8=½. 8(8+21)
=4.29=116
No. 13
U4=7
a+6b=7
a=7-6b
S10=145
½.10((2(7-6b) +9b)) =145
5(14-12b+9b) =145
14-3b=29
3b=14-29
3b=-15
b=-5
a=7-6b=7-6(-5)=7+30=37
S12=½. 12(74-55)
=6.19=114
No. 14
S12=246
U1+U2+U3=21
a+a+b+a+2b=21
3a+3b=21
3(a+b) =21
a+b=7
a=7-b
S12=½. 12((2(7-b) +11b)) =246
6(14-2b+11b) =246
14+9b=41
9b=41-14
9b=27
b=3
a=7-3=4
U12=a+11b=4+11.3=4+33=37
Jd potongan tali terpanjang=37 cm