evaluate (without using table) : b) cos13°×cosec77°
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So we know,
Sin A = cos (90-A)
so in cos 13 we can write it as sin (90-13)
= sin 77
and cosec A = 1/ sin a
So
cos 13 × cosec 77
= sin 77 × 1/sin 77
= 1
So , cos13°×cosec77° = 1
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