Solve for x : 4sinx - 3cosx = 0

2 Answers | Add Yours

william1941's profile pic

william1941 | College Teacher | (Level 3) Valedictorian

Posted on

We have to solve 4sinx - 3cosx = 0

Now 4 sin x - 3 cos x =0

=> 4 sin x = 3 cos x

divide by 4 cos x

=> 4 sin x / 4 cos x = 3 cos x / 4 cos x

=> tan x = 3/4

Therefore x = arc tan (3/4)

or x= 36.86 + n*180 degrees. (where n is any integer)

giorgiana1976's profile pic

giorgiana1976 | College Teacher | (Level 3) Valedictorian

Posted on

We'll solve the problem using 2 methods

First method:

4 sin x = 3 cos x

sin x =( 3/4) cos x

We'll divide by cos x:

sinx/cosx = 3/4

But the ratio sinx/cosx = tan x

tan x= 3/4

x = arctan (3/4) + k*pi

The second method:

We know that in a right triangle, due to Pythagorean theorem,

sin^2 x + cos^2 x = 1

sin x = sqrt[1 - cos^2 (x)]

But, from hypothesis, sin x = (3/4)cos x,so

(3/4)cos (x) = sqrt[1 - cos^2 (x)]

We'll square raise both sides:

[(3/4)cos (x)]^2 = {sqrt[1 - cos^2 (x)]}^2

(9/16)cos^2 (x)= 1 - cos^2 (x)

(9/16)cos^2 (x )+ cos^2 (x) = 1

The least common denominator is 16, so we'll multiply with 16, cos^2 (x) and the result will be:

(25/16)cos^2 (x) = 1

cos^2 (x) = 16/25

cos x = 4/5

x = arccos (4/5) + 2*k*pi

We’ve answered 318,917 questions. We can answer yours, too.

Ask a question