tolong jawab..... terima kasih

Un=3n-5
a) U1=3×1-5=3-5
suku pertama (a) =-2

b) beda (b)= U2-U1=b
(3×2-5)-(-2)=b
(6-5)+2=b
b=1+2=3

c) Sn=n[2a+(n-1)b] ÷ 2
S7=7 [2.-2 + (7-1) 3] ÷ 2
S7=7 [-4 + 6 x 3] ÷ 2
S7=7 [-4 + 18] ÷ 2
S7= 7 x 14 ÷ 2
S7= 7 x 7
S7= 49

a) Un = 3n - 5
     U₁ = 3(1) - 5
      a  = - 2
    
b) U₂ = 3(2) - 5
         = 1
    U₂ = a + (n-1)b
     1  = - 2 + (2 - 1)b
      1 = - 2 + b
      b = 3

c) S₇ = 7/2 ( a + Un )
         = 7/2 ( -2 + (3 (7) - 5) )
         = 7/2 (14)
         = 49