The values of x over the interval 0° < x < 180° for which cos(3x) = 1 are

The solution is x = 120º

This question is the solution of a trigonometric equation, involving cosine.

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Standard equation:

has the solutions given by:

In which n = 0, 1, 2,...

Considering that , the solution, in degrees, is:

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In this question:

Thus, a = 3, and the solution is:

We want solutions greater than 0º and less than 180º, so the solution is x = 120º.

A similar example is found at link

c. y = 1/4 x + 5

step-by-step explanation:

first, put the equation of the line on the graph into slope-intercept form

(y = mx + b):   y = -4x + 8

second, a perpendicular line has to have "an opposite/reciprocal slope" to the original slope. so, find the "m" value in your original equation (it's -4, right? )

original slope: m = -4   perpendicular slope: change the sign to its opposite and find the reciprocal ("flip" the number)

new slope is now positive (b/c the original slope was negative 4), and it's 1/4 (not the original slope of 4 or 4/1)