tolong, besok dikumpulkan

a. Un = a + (n-1)b = 5 + (n-1)3 = 5 + 3n - 3 = 3n + 2. 
    U20 = 3 (20) + 2 = 62. 
b. Un = a + (n-1)b = [tex] \frac{-21}{2} [/tex] + (n-1)[tex] \frac{5}{2} [/tex]
         =[tex] \frac{-21}{2} - \frac{5}{2} + \frac{5n}{2} = \frac{-16}{2} + \frac{5n}{2} = -8 - \frac{5n}{2} [/tex]

U20 = -8 +5(20)/2 = -8 + 50 = 42. 
c. Un = a + (n-1)b = 35 + (n-1)(-4) = 35 -4n + 4 = 39 - 4n. 
U20 = 39 -80 = -41.