No.8 gimana?ga ngerti wkwkwk

Bab Differensial
Matematika SMA Kelas XI

f(x) = x . cos 2x
f'(x) = 1 . cos 2x + (x . (-sin 2x) . 2
f'(x) = cos 2x - 2x sin 2x
f'(-π/4) = cos (2 . -π/4) - 2 (-π/4) . sin (2 . -π/4)
f'(-π/4) = 0 + π/2 . (-1)
f'(-π/4) = -π/2

jawabannya A