tolong bantu nomor 12 sama 13 aku gak paham , dapet tapi gk ada jawabannya

13. F(x) = -2/3x³ + 2x + 2/3
F'(x) = -2x² + 2
Syarat stasioner, f'(x) = 0
-2x² = -2
x = ±1
F(2) = -2/3(2)³ + 2(2) + 2/3 = -16/3 + 4 + 2/3 = -14/3 + 4 = -2/3 (minimum)
F(1) = -2/3 + 2 + 2/3 = 2
F(-1) = 2/3 - 2 + 2/3 = 4/3-2 = -2/3
Jadi, nilai deret tak hingganya 2.
Suku pertama : 1/2f'(0) = 1/2(2)= 1/4
2 = 1/4/(1 - r)
2 - 2r = 1/4
-2r = 1/4 - 2 = -7/4
r = 7/8
Jawab : A.
Koreksi lg ya??