6. Expressing a sin θ ± b cos θ in the form R sin(θ ± α)

by M. Bourne

In electronics, we often get expressions involving the sum of sine and cosine terms. It is more convenient to write such expressions using one single term.

Our Problem:

Express a sin θ ± b cos θ in the form

R sin(θ ± α),

where a, b, R and α are positive constants.

Solution:

First we take the "plus" case, (θ + α), to make things easy.

Let

a sin θ + b cos θ ≡ R sin (θ + α)

(The symbol " ≡ " means: "is identically equal to")

Using the compound angle formula from before (Sine of the sum of angles),

sin(A + B) = sin A cos B + cos A sin B,

we can expand R sin (θ + α) as follows:

R sin (θ + α)

≡ R (sin θ cos α + cos θ sin α)
≡ R sin θ cos α + R cos θ sin α

a sin θ + b cos θ ≡ R cos α sin θ + R sin α cos θ

Equating the coefficients of sin θ and cos θ in this identity, we have:

For sin θ: a = R cos α ..........(1) in green above

For cos θ: b = R sin α .........(2) in red above

Eq. (2) ÷ Eq.(1):

(α is a positive acute angle and a and b are positive.)

Now we square each of Eq. (1) and Eq. (2) and add themm to find an expression for R.

[Eq. (1)]² + [Eq. (2)]²:

a² + b²

= R² cos² α + R² sin² α

= R²(cos² α + sin² α)

= R² (since cos² A + sin² A = 1)

(We take only the positive root)

In summary, if

and

then we have expressed a sin θ + b cos θ in the form required:

a sin θ + b cos θ = R sin(θ + α)

You will notice that this is very similar to converting rectangular to polar form in Polar form of Complex Numbers. We can get α and R using calculator, similar to the way we did it in the complex numbers section.

Similarly, for the minus case, we equate a sin θ − b cos θ with the expansion of R sin (θ − α) as follows (note the minus signs carefully):

a sin θ − b cos θ ≡ R cos α sin θ − R sin α cos θ

Once again we will obtain (try it yourself!):

and

Our equation for the minus case is:

a sin θ − b cos θ = R sin(θ − α)

Equations of the type a sin θ ± b cos θ = c

To solve an equation in the form

a sin θ ± b cos θ = c,

express the LHS in the form R sin(θ ± α) and then solve

R sin(θ ± α) = c.

Exercises - Sine Form

1. (a) Express 4 sin θ + 3 cos θ in the form R sin(θ + α).

(b) Using your answer from part (a), solve the equation

4 sin θ + 3 cos θ = 2 for 0° ≤ θ < 360°.

Answer

2. Solve the equation

for 0° ≤ θ < 360°.

Answer

3. The current i (in amperes) at time t in a particular circuit is given by

i = 12 sin t + 5 cos t.

Find the maximum current and the first time that it occurs.

Answer

4. Solve 7 sin 3θ − 6 cos 3θ = 3.8 for 0° ≤ θ < 360°.

Answer

5. The current i amperes in a certain circuit after t seconds is given by

math .

Find the maximum current and the earliest time it occurs.
(Note: t > 0)

Answer

The Cosine Form

We can also express our sum of a sine term and a cosine term using cosine rather than sine. It may be more convenient to do so in some situations.

The expressions obtained are similar to those we obtained for the sine case, but note the differences as we proceed.

For a, b and R positive and α acute, our equivalent expression is given by: