Mathematics — Trigonometry for the JEE Main Exam — Chinese candidates

Common failure modes

These are the patterns that cause most candidates to lose marks on this topic. Recognising them in advance is half the work.

• !Forgetting the domain and range of arcsin (−π/2 to π/2) vs arccos (0 to π)
• !Applying sum-to-product formulas incorrectly when signs differ (sin A − sin B ≠ sin(A−B))
• !Missing multiple solutions of trigonometric equations by not writing the general solution
• !Confusing the law of sines (a/sin A) with the law of cosines — choosing the wrong formula for the given data

Study tips

• 1Memorise the transformation identities (sum-to-product, product-to-sum) — they appear in definite integral evaluations as well as pure trigonometry questions.
• 2For inverse trig, drill the composition identities: sin(arccos x) = √(1−x²), arcsin(x) + arccos(x) = π/2, and the double-angle inverses.
• 3Write the general solution for trigonometric equations every time: sin θ = k → θ = nπ + (−1)ⁿ arcsin k. JEE marks are lost by giving only one particular solution.
• 4Practice properties-of-triangles (sine rule, cosine rule, area formula) timed at 2 minutes per problem.
• 5中国考生备考 JEE Main 时，建议优先攻克英语听力与写作两个最易失分的板块 — 每日固定时段做真题模拟。

Sample JEE Main Mathematics — Trigonometry questions

These sample items mirror the format and difficulty of real JEE Main questions. Practice with thousands more on the free Koydo question bank.

1. 1

   The principal value of arcsin(−1/2) is:

   • A−π/6Correct
   • Bπ/6
   • C−π/3
   • D5π/6
   Why this answer?

   Illustrative JEE-style: sin(−π/6) = −1/2, and −π/6 lies in the principal range [−π/2, π/2]. Therefore arcsin(−1/2) = −π/6.

2. 2

   If sin A + sin B = x and cos A + cos B = y, then tan((A+B)/2) equals:

   • Ax/yCorrect
   • By/x
   • C(x+y)/(x−y)
   • D(x−y)/(x+y)
   Why this answer?

   Illustrative JEE-style: Using sum-to-product: sin A + sin B = 2 sin((A+B)/2) cos((A−B)/2) = x, and cos A + cos B = 2 cos((A+B)/2) cos((A−B)/2) = y. Dividing gives tan((A+B)/2) = x/y.

3. 3

   In a triangle, if a = 2, b = 3, and C = 60°, the value of side c (by the cosine rule) is:

   • A√7Correct
   • B√13
   • C√5
   • D√19
   Why this answer?

   Illustrative JEE-style: c² = a² + b² − 2ab cos C = 4 + 9 − 2(2)(3)(1/2) = 13 − 6 = 7. So c = √7.

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