Take cos(2x) = 1/√2. solve for x (in terms of radians). i already have pi/8 and 15pi/8, but i know there are two other answers. where do those answers come from?

Take the arccos of your equation.

... arccos(cos(2x)) = arccos(1/√2)

... 2x = {π/4+2kπ, -π/4+2kπ} . . . . list all solutions

... x = {π/8+kπ, -π/8+kπ}

... x = {π/8, 7π/8, 9π/8, 15π/8}

In general, when x has a multiplier, you need to consider angles in the range of 0 to that multiple of 2π where you might normally only look in the range [0, 2π).

based on context clues you gave i'm assuming the answer would be c.

ok

step-by-step explanation: