I need to prove this identity and tried changing the cotA's to 1/tanA etc. but it didn't really help.
cotA-tanA≡2cot2A.
Please tell me what an earth (or sea) is going on here!
Thanks
Ahh I see! Thankyou
Try changing to sines and cosines.
cosA/sinA - sinA/cosA = 2cos2A/sin2A
cos2A = cos2A - sin2A
sin2A = 2sinAcosA
so, right-hand side is 2cos2A/sin2A = 2(cos2A - sin2A)/(2sinAcosA) = cos2A/(sinAcosA) - sin2A/(sinAcosA) = cosA/sinA - sinA/cosA = left-hand side.
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