Question

Find the value of cos69°45' - cos15°40'.
(Refer the trigonometric table for the values)

• A. - 0.6495
• B. 0.6495
• C. - 0.6618
• D. 0.6618

Answer 1

The correct option is A - 0.6495

We can write,
69°45' = 69°42' + 3'
From the table we have, cos69°42' = 0.3469
Mean difference for, 3′ = 8

cos69°42' = 0.3469
3′ = - 8 (Mean difference to be subtracted for cosine)
____________________
cos69°45' = 0.3461 ...(i)


We can write,
15°40' = 15°36' + 4'
From the table we have, cos15°36' = 0.9632
Mean difference for, 4′ = 3
cos15°36' = 0.9632
4′ = - 3 (Mean difference to be subtracted for cosine)
____________________
cos15°40' = 0.9629 ...(ii)

Solving equation (i) - (ii), we get
cos69°45' - cos15°40' = 0.3461 - 0.9629 = - 0.6168