di ketahui deret aritmatika u1=3 dan u5=-5 maka jumlah 10 suku pertama adalah

U1 = 3 = a
U5 = -5 = a+4b

a+4b = -5
3+4b = -5
4b = -8
b = -2

Sn = n/2×(2a+(n-1)b)
S10 = 10/2 ×(2×3 + (10-1) × -2)
S10 = 5 × (6+(-18))
S10 = 5 × (-12)
S10 = -60

Un = a + (n - 1)

U1 = 3
U5 = a + 4b
-5 = 3 + 4b
-5 - 3 = 4b
-8 = 4b
-8/4 = b
-2 = b

Sn = n/2 (2a + (n - 1)b)
S10 = 10/2 (2.3 + 9.(-2))
S10 = 5(6 + (-18))
S10 = 5 . -12
S10 = -60

Semoga membantu dan semoga bermanfaat...