Question Number 157686 by MathSh last updated on 26/Oct/21
$$\mathrm{if}\:\:\:\mathrm{f}\left(\mathrm{x}+\mathrm{1}\right)-\mathrm{f}\left(\mathrm{x}\right)=\mathrm{3}\:\:\:\mathrm{and}\:\:\:\mathrm{f}\left(\mathrm{25}\right)=\mathrm{72} \\ $$$$\mathrm{find}\:\:\:\mathrm{f}\left(\mathrm{2}\right)\:=\:? \\ $$
Answered by tounghoungko last updated on 26/Oct/21
$$\Rightarrow{f}\left({x}\right)={f}\left({x}+\mathrm{1}\right)−\mathrm{3} \\ $$$$\Rightarrow{f}\left(\mathrm{25}\right)={f}\left(\mathrm{24}\right)−\mathrm{3} \\ $$$$\Rightarrow{f}\left(\mathrm{24}\right)={f}\left(\mathrm{23}\right)−\mathrm{3} \\ $$$$\:\:\:\:\:\vdots \\ $$$$\Rightarrow{f}\left(\mathrm{3}\right)={f}\left(\mathrm{2}\right)−\mathrm{3} \\ $$$$−−−−−−−−−−− \\ $$$$\Rightarrow{f}\left(\mathrm{25}\right)=\mathrm{23}×\left(−\mathrm{3}\right)+{f}\left(\mathrm{2}\right) \\ $$$$\Rightarrow{f}\left(\mathrm{2}\right)=\mathrm{72}+\mathrm{69}=\mathrm{141} \\ $$
Answered by mr W last updated on 26/Oct/21
$${f}\left({x}+\mathrm{1}\right)={f}\left({x}\right)+\mathrm{3} \\ $$$${f}\left({x}+\mathrm{2}\right)={f}\left({x}+\mathrm{1}\right)+\mathrm{3}={f}\left({x}\right)+\mathrm{2}×\mathrm{3} \\ $$$$… \\ $$$${f}\left({x}+{n}\right)={f}\left({x}\right)+\mathrm{3}{n} \\ $$$$ \\ $$$${f}\left(\mathrm{25}\right)={f}\left(\mathrm{2}\right)+\mathrm{3}\left(\mathrm{25}−\mathrm{2}\right) \\ $$$${f}\left(\mathrm{2}\right)={f}\left(\mathrm{25}\right)−\mathrm{3}×\mathrm{23}=\mathrm{72}−\mathrm{3}×\mathrm{23}=\mathrm{3} \\ $$
Commented by MathSh last updated on 26/Oct/21
$$\mathrm{Very}\:\mathrm{nice}\:\mathrm{dear}\:\boldsymbol{\mathrm{S}}\mathrm{er},\:\mathrm{thank}\:\mathrm{you}\:\mathrm{so}\:\mathrm{much} \\ $$
Answered by qaz last updated on 26/Oct/21
$$\mathrm{f}\left(\mathrm{x}+\mathrm{1}\right)−\mathrm{f}\left(\mathrm{x}\right)=\mathrm{3} \\ $$$$\underset{\mathrm{k}=\mathrm{x}} {\overset{\mathrm{n}} {\sum}}\left(\mathrm{f}\left(\mathrm{k}+\mathrm{1}\right)−\mathrm{f}\left(\mathrm{k}\right)\right)=\underset{\mathrm{k}=\mathrm{x}} {\overset{\mathrm{n}} {\sum}}\mathrm{3} \\ $$$$\Rightarrow\mathrm{f}\left(\mathrm{n}+\mathrm{1}\right)−\mathrm{f}\left(\mathrm{x}\right)=\mathrm{3}\left(\mathrm{n}+\mathrm{1}−\mathrm{x}\right) \\ $$$$\mathrm{f}\left(\mathrm{25}\right)−\mathrm{f}\left(\mathrm{2}\right)=\mathrm{3}\left(\mathrm{25}−\mathrm{2}\right)=\mathrm{69} \\ $$$$\Rightarrow\mathrm{f}\left(\mathrm{2}\right)=\mathrm{72}−\mathrm{69}=\mathrm{3} \\ $$
Commented by MathSh last updated on 26/Oct/21
$$\mathrm{Very}\:\mathrm{nice}\:\mathrm{dear}\:\boldsymbol{\mathrm{S}}\mathrm{er},\:\mathrm{thank}\:\mathrm{you}\:\mathrm{so}\:\mathrm{much} \\ $$