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In triangle ABC, right-angled at B, if tan A = 1/√3 find the value of: (ii) cos A cos C – sin A sin C. Q.9(2)

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Find the best way to solve ncert class 10 introduction to trigonometry question . How i solve the exercise 8.1 question no.9(2) easily and quickly  because its very important question for this chapter.In triangle ABC, right-angled at B, if tan A = 1/√3 find the value of: (ii) cos A cos C – sin A sin C.

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  1. Let ΔABC in which ∠B=90°

    tan A = BC/AB = 1/√3

    Let BC = 1k and AB = √3 k,

    Where k is the positive real number of the problem

    By Pythagoras theorem in ΔABC we get:

    AC2=AB2+BC2

    AC2=(√3 k)2+(k)2

    AC2=3k2+k2

    AC2=4k2

    AC = 2k

    Now find the values of cos A, Sin A

    Sin A = BC/AC = 1/2

    Cos A = AB/AC = √3/2

    Then find the values of cos C and sin C

    Sin C = AB/AC = 3/2

    Cos C = BC/AC = 1/2

    Now, substitute the values in the given problem

    (i) sin A cos C + cos A sin C = (1/2) ×(1/2 )+ √3/2 ×√3/2 = 1/4 + 3/4 = 1

    (ii) cos A cos C – sin A sin C = (3/2 )(1/2) – (1/2) (3/2 ) = 0

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