thakx

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Tan50 = tan(40+10)
we have tan(a+b) = (tan a + tan b) / (1-tan a tan b)
tan(40+10) = (tan 40 + tan10) / (1-tan 40 tan 10)
tan50 = (tan40 + tan 10) / (1- tan40 tan 10)
tan50 (1-tan40 tan 10) = tan40 + tan10
tan50 - tan50 tan40 tan10 = tan40 + tan10
and tan50 = tan(90-40) = cot40
=> tan50 - cot40 tan40 tan10 = tan40 + tan10
tan50 - tan10 = tan40 + tan10
because cot40 = 1/tan40
tan50 = tan40 + 2tan10