# If sinθ + cosθ = p and sec θ + cosec θ = q,

Question:

If sinθ + cosθ = p and sec θ + cosec θ = q, then prove that q(p2-1) = 2p.

Solution:

Given that, $\quad \sin \theta+\cos \theta=p$ $\ldots(i)$

and $\quad \sec \theta+\operatorname{cosec} \theta=q$

$\Rightarrow$ $\frac{1}{\cos \theta}+\frac{1}{\sin \theta}=q$ $\left[\because \sec \theta=\frac{1}{\cos \theta}\right.$ and $\left.\operatorname{cosec} \theta=\frac{1}{\sin \theta}\right]$

$\Rightarrow$ $\frac{\sin \theta+\cos \theta}{\sin \theta \cdot \cos \theta}=q$

$\Rightarrow$ $\frac{p}{\sin \theta \cdot \cos \theta}=q$ [from Eq. (i)]

$\Rightarrow$ $\sin \theta \cdot \cos \theta=\frac{p}{q}$ [from Eq. (i)]...(ii)

$\sin \theta+\cos \theta=p$

On squaring both sides, we get

$(\sin \theta+\cos \theta)^{2}=p^{2}$

$\Rightarrow \quad\left(\sin ^{2} \theta+\cos ^{2} \theta\right)+2 \sin \theta \cdot \cos \theta=p^{2} \quad\left[\because(a+b)^{2}=a^{2}+2 a b+b^{2}\right]$

$\Rightarrow \quad 1+2 \sin \theta \cdot \cos \theta=p^{2}$ $\left[\because \sin ^{2} \theta+\cos ^{2} \theta=1\right]$

$\Rightarrow$ $1+2 \cdot \frac{p}{q}=p^{2}$ [from Eq. (ii)]

$\Rightarrow$ $q+2 p=p^{2} q \Rightarrow 2 p=p^{2} q-q$

$\Rightarrow$ $q\left(p^{2}-1\right)=2 p$

Hence proved.

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