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If (cos(A-B))/("cos"(A+B))+("cos"(C+D))/...

If `(cos(A-B))/("cos"(A+B))+("cos"(C+D))/("cos"(C-D))=0,\ ` prove tht `tanAtanBtanCtanD=-1`

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Given,

`(cos ({A}-{B}) / cos ({A}+{B}))+(cos ({C}+{D}) / cos ({C}-{D}))=0 `

`Rightarrow frac{cos (A-B)}{cos (A+B)}=-frac{cos (C+D)}{cos (C-D)}`

Adding 1 both sides:

...
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