B, because when you add 90+80 you get 170 and 170/2 = 85
Negative pi over 5
− π/ 5
Assuming you mean f(t) = g(t) × h(t), notice that
f(t) = g(t) × h(t) = cos(t) sin(t) = 1/2 sin(2t)
Then the difference quotient of f is

Recall the angle sum identity for sine:
sin(x + y) = sin(x) cos(y) + cos(x) sin(y)
Then we can write the difference quotient as

or

(As a bonus, notice that as h approaches 0, we have (cos(2h) - 1)/(2h) → 0 and sin(2h)/(2h) → 1, so we recover the derivative of f(t) as cos(2t).)
0= -3x+9
-9=-3x
3=x
Final answer: x=3
There is only one zero because it is linear, and can therefore only cross the x axis once.