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Trigonometric Functions — Maths STD 11 Science — Question

CBSE BoardEnglish MediumSTD 11 ScienceMathsTrigonometric Functions1 Mark
MCQ
The value of $2\sin^2\text{B}+4\cos(\text{A+B})\sin\text{A}\sin\text{B}+\cos^2(\text{A+B})$ is:
  • A
    $0$
  • B
    $\cos3\text{A}$
  • ✓
    $\cos2\text{A}$
  • D
    None of these
✓

Answer

Correct option: C.
$\cos2\text{A}$
We have,
$2\sin^2​\text{B}+4\cos(\text{A+B})\sin​​\text{A}\sin​\text{B}+\cos2​\text{A+B}$
$=1-\cos2​\text{B}+\cos^2(​\text{A+B})+4\cos(​\text{A+B})\sin​\text{A}\sin​\text{B}$
$=1+(\cos2(​\text{A+B})-\cos2​\text{B})+4\cos(​\text{A+B})\sin​\text{A}\sin​\text{B}$
$=1-2\sin​\text{A}\sin(​\text{A+2B})+4\cos(​\text{A+B})\sin​\text{A}\sin​\text{B}\\ \ \ \ \ \ \ \ \ \Big[\because\cos​\text{C}\cos​\text{D}=-2\sin\frac{​\text{C+D}}{2}\sin\frac{​\text{C}-​\text{D}}{2}\Big]$
$=1-2\sin​\text{A}[\sin(​\text{A}+2​\text{B})-2\sin​\text{B}-2\cos(​\text{A+B})]$
$=1-2\sin​\text{A}[\sin(​\text{A}+2​\text{B})-\big\{\sin(\text{B+A+B})+\sin(\text{B}-\text{(A+B)})\big\}\\ \ \ \ \ \ [\because2\sin\text{C}\cos\text{D}=\sin(\text{C+D})+\sin(\text{C}-\text{D})]$
$=1-2\sin\text{A}\big[\sin(\text{A+2B})-\big\{\sin(\text{A+2B})+\sin(-\text{A})\big\}\big]$
$=1-2\sin\text{A}[\sin\text{A}]$
$=1-2\sin2\text{A}$
$=\cos2\text{A}$

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