-64 + -4 = -68
That is because 64 + 4 = 68 and the negatives in this case are the same way as if it was all positive.
F(x) = cos x
f'(x) = -sin x
f''(x) = -cos x
f'''(x) = sin x
f''''(x) = cos x
90/4 = 22 Remainder 2
Therefore, 90th derivative of f(x) = cos x is -cos x.
65-14=65-5-x
x=9
Explanation=
65-14=65-5-x
51=60-x
-9=-x
9=x