HSC Ext 1 Math Australia trigonometry question 

Prove that $\sin^2(\pi/18)+\sin^2(5\pi/18)+\sin^2(25\pi/18)=3/2$.

I believe the way im meant to solve this involves mainly the use of tried identities, and I have a strong feeling the triple angle identities have a part. Also this is a band 4 question worth 3 marks


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  • Can I ask what Identity you used in the first step to convert sin^2(π/18) to (1-cos(π/9))/2?

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