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Question Number 103458 by bobhans last updated on 15/Jul/20

(1)If p and q are the roots quadratic  equation x^2 −x−4050 = 0 , find the value  of p^2 +q ?   (2) if f(x)=((7x+q)/2) and g(x)=7x+8 ;  (fog)(4) = 24 . find the value of q

$$\left(\mathrm{1}\right){If}\:{p}\:{and}\:{q}\:{are}\:{the}\:{roots}\:{quadratic} \\ $$$${equation}\:{x}^{\mathrm{2}} −{x}−\mathrm{4050}\:=\:\mathrm{0}\:,\:{find}\:{the}\:{value} \\ $$$${of}\:{p}^{\mathrm{2}} +{q}\:?\: \\ $$$$\left(\mathrm{2}\right)\:{if}\:{f}\left({x}\right)=\frac{\mathrm{7}{x}+{q}}{\mathrm{2}}\:{and}\:{g}\left({x}\right)=\mathrm{7}{x}+\mathrm{8}\:; \\ $$$$\left({fog}\right)\left(\mathrm{4}\right)\:=\:\mathrm{24}\:.\:{find}\:{the}\:{value}\:{of}\:{q} \\ $$

Answered by bemath last updated on 15/Jul/20

(1) x^2 −x−4050=0→ { (p),(q) :}  ⇔p^2  = p+4050  then p^2 +q = p+4050+q  = p+q +4050  = 1+4050 = 4051 ;(by Vieta′s  rule )★

$$\left(\mathrm{1}\right)\:{x}^{\mathrm{2}} −{x}−\mathrm{4050}=\mathrm{0}\rightarrow\begin{cases}{{p}}\\{{q}}\end{cases} \\ $$$$\Leftrightarrow{p}^{\mathrm{2}} \:=\:{p}+\mathrm{4050} \\ $$$${then}\:{p}^{\mathrm{2}} +{q}\:=\:{p}+\mathrm{4050}+{q} \\ $$$$=\:{p}+{q}\:+\mathrm{4050} \\ $$$$=\:\mathrm{1}+\mathrm{4050}\:=\:\mathrm{4051}\:;\left({by}\:{Vieta}'{s}\right. \\ $$$$\left.{rule}\:\right)\bigstar \\ $$

Answered by bemath last updated on 15/Jul/20

(2)(fog)(4)= f(g(4))  ⇒g(4)= 7.4 + 8 = 36   ⇒f(36) = 24 →((7.36+q)/2) = 24  q = 48−252 = −204 ★

$$\left(\mathrm{2}\right)\left({fog}\right)\left(\mathrm{4}\right)=\:{f}\left({g}\left(\mathrm{4}\right)\right) \\ $$$$\Rightarrow{g}\left(\mathrm{4}\right)=\:\mathrm{7}.\mathrm{4}\:+\:\mathrm{8}\:=\:\mathrm{36}\: \\ $$$$\Rightarrow{f}\left(\mathrm{36}\right)\:=\:\mathrm{24}\:\rightarrow\frac{\mathrm{7}.\mathrm{36}+{q}}{\mathrm{2}}\:=\:\mathrm{24} \\ $$$${q}\:=\:\mathrm{48}−\mathrm{252}\:=\:−\mathrm{204}\:\bigstar\: \\ $$

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