tentukan banyak sukunya U3+U5+U7=15 U2+u3+u4=3 Jika Un=31,Nilai dari n adalah

U3+U5+U7=15
a + 2b + a + 4b + a + 6b = 15
3a + 10b = 15

U2+u3+u4=3
a + b + a + 2b + a + 3b = 3
3a + 6b = 3
a + 2b = 1
a = 1 - 2b

3a + 10b = 15
3(1 - 2b) + 10b = 15
3 - 6b + 10b = 15
3 + 4b = 15
4b = 15 - 3
4b = 12
b = 12/4
b = 3

a = 1 - 2b
a = 1 - 2(3)
a = 1 - 6
a = -5

Un = a + (n-1)b
31 = (-5) + (n-1)(3)
31 + 5 = 3n - 3
36 + 3 = 3n
39 = 3n
39/3 = n
n = 13