Quick Trig question? (1 Viewer)

LoveHateSchool · Sep 6, 2011

I need to show that (sintheta +costheta)(sintheta-costheta) =2sin^2theta-1

I must be totally braindead because it looks like it should be easier to prove but I can't

SpiralFlex · Sep 6, 2011

$LHS=(\sin \theta + \cos \theta)(\sin \theta-\cos \theta)$

$=\sin^2 \theta - \cos^2 \theta$ [Difference between two squares.]

$=\sin^2\theta - (1- \sin^2 \theta)$ [Since $\sin^2 \theta + \cos^2 \theta =1$ ]

$=2\sin^2 \theta -1$

$=RHS$

Keep practising, you will get this question instantly with more practice as this is a recognisable double angle for cosine.

LoveHateSchool · Sep 7, 2011

Thank you, I expanded the bracket wrong and that's why it wasn't working! It always tells me to spread rep, access denied for giving rep to the amazing robot.

Doing past papers, and Trig is definitely my number one topic to revise thoroughly for yearly exam.

SpiralFlex · Sep 7, 2011

LoveHateSchool said:
Thank you, I expanded the bracket wrong and that's why it wasn't working! It always tells me to spread rep, access denied for giving rep to the amazing robot.

Doing past papers, and Trig is definitely my number one topic to revise thoroughly for yearly exam.

You should note it's a difference between two squares. Enjoy human.

Quick Trig question? (1 Viewer)

LoveHateSchool

Retired Sept '14

SpiralFlex

Well-Known Member

LoveHateSchool

Retired Sept '14

SpiralFlex

Well-Known Member

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