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March 5, 2015

March 5, 2015

Posted by **aayush** on Friday, January 25, 2008 at 12:47pm.

prove that:

2tan(x-y)=tan(x+y)

( here, in sin2x, 2x is an angle., like there's a formula:sin2x=2sinxcosx and sin2y=2sinycosy; ....)

- trigonometry -
**drwls**, Friday, January 25, 2008 at 1:17pmIt cannot be proven because it is not true. You can prove that by choosing x and y that satisfies sin2x=3sin2y. For example, take x = 15 degrees and y = 9.5941 degrees.

For those choices, sin 2x = 3sin 2y = 0.500

For those choices of x and y

2 tan(x-y) = 0.1893 and tan (x+y) = 0.4577

By being able to find ANY x,y combination that satisfies sin2x=3sin2y, and for which your second equation is not true, then your contention is disproved.

Either someone is playing a practical joke on us, or on you, or you have copied the problem incorrectly

- trigonometry (correction) -
**drwls**, Friday, January 25, 2008 at 6:02pmI made a mistake in my example. Choose the angles x = 15 degrees and y = 4.79703 degrees. Then the condition

sin 2x = 3 sin 2y is satisfied.

2tan(x-y) = 0.35996

tan (x+y) = 0.35996

Your identity is almost certainly correct and I must apologize for my error in the previous answer.

I tried to prove the second equation for any x and y that satisfy your first equation, but was not able to.

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