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Question

If cos135sin145=cos1x, then x is equal to -

A
0
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B
1
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C
-1
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D
None of these
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Solution

The correct option is C None of these
Given,

cos135+sin145=cos1x

apply cos on both sides, we get,

cos(cos135+sin145)=cos(cos1x)

Usethefollowingidentity:cos(s+t)=cos(s)cos(t)sin(s)sin(t)

cos(cos1(35))cos(sin1(45))sin(cos1(35))sin(sin1(45))=x

351(45)2451(35)2=x

35×3545×45=x

9251625=x

x=725

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