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c3 trig help

solve sin theta+cos theta=1

i have not done anything...and chapter 7 of edexcel c3 book
Original post by coolgamer
solve sin theta+cos theta=1

i have not done anything...and chapter 7 of edexcel c3 book



Try squaring both sides and applying the rule sin^2theta + cos^2theta = 1 and the rule 2(sin theta)(cos theta) = sin2theta
then just find the value of theta, the usual CAST quadrant way.
Reply 2
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TeeEm
19
Original post by chemlover12
Try squaring both sides and applying the rule sin^2theta + cos^2theta = 1 and the rule 2(sin theta)(cos theta) = sin2theta
then just find the value of theta, the usual CAST quadrant way.



Original post by coolgamer
solve sin theta+cos theta=1

i have not done anything...and chapter 7 of edexcel c3 book



squaring will work but it is a "dangerous" approach as it will create extra solutions.

The standard way is to use R-transformation, i.e. Rsin(x+a) or Rcos(x-a)
Reply 3
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davros
16
Original post by coolgamer
solve sin theta+cos theta=1

i have not done anything...and chapter 7 of edexcel c3 book


The standard way to solve this is to write the LHS in harmonic form: Rsin(theta + a) for suitable values of R and a.

(If you're familiar with the identity for sin(A+B) you can almost write down what a and R have to be!)
Original post by davros
The standard way to solve this is to write the LHS in harmonic form: Rsin(theta + a) for suitable values of R and a.

(If you're familiar with the identity for sin(A+B) you can almost write down what a and R have to be!)


this is also called Wave Form ?
Reply 5
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coolgamer
OP
11
it says multiply each term by 1/root2
Reply 6
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davros
16
Original post by coolgamer
it says multiply each term by 1/root2


Well that's a start!

When you've done that you should immediately recognize the LHS as sin(theta + a) for some well-known value of a!
Reply 7
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coolgamer
OP
11
it was a hint in the book....how are u supposed to know when to do something like this? when there is no hint in the exam
Original post by coolgamer
it was a hint in the book....how are u supposed to know when to do something like this? when there is no hint in the exam


Whats the point of the exam if you're going to get hints and tips, you should be able to recognise that acos(theta) + bcos(theta ) can be written in harmonic form.
Reply 9
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davros
16
Original post by coolgamer
it was a hint in the book....how are u supposed to know when to do something like this? when there is no hint in the exam


As suggested earlier, once you've learned how to convert things like asinx + bcosx into harmonic form Rsin(x + t) then you'll have a method for dealing with general questions like this.
Original post by TeeEm
squaring will work but it is a "dangerous" approach as it will create extra solutions.

The standard way is to use R-transformation, i.e. Rsin(x+a) or Rcos(x-a)


Oh yeah. Thanks :smile:
Reply 11
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TeeEm
19
Original post by chemlover12
Oh yeah. Thanks :smile:


pleasure
Reply 12
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coolgamer
OP
11
thanks all can i have some tips for trig

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