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Question

If tanα = mm+1 and tanβ = 12m+1, then α + β =

[IIT 1978; EAMCET 1992; Roorkee 1998; JMI EEE 2001]


A

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B

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C

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D

None of these

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Solution

The correct option is B


We have, tan α = mm+1 and tan β = 12m+1

We know tan(α+β) = tanα+tanβ1tanαtanβ

= mm+1+12m+11m(m+1)1(2m+1) = 2m2+m+m+12m2+m+2m+1m

= 2m2+2m+12m2+2m+1 = 1 tan(α + β) = tan π4

Hence, α+β = π4.

Trick: As α+β is independent of m, therefore put m = 1,

then tanα = 12 and tanβ = 13. Therefore,

tan(α+β) = (12)+131(16) = 1. Hence α + β = π4.

(Also check for other values of m).


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