Question #307682

(a) Use Euclidean algorithm to find the gcd of 105 and 231.


(b) Use mathematical induction to show that


1 · 1! + 2 · 2! + · · · + n · n! = (n + 1)! − 1


(c) Show that the relation ∼ defined on R as a ∼ b if b−a ∈ Q, is an equivalence relation.


Also, find the equivalence class of 1.


1
Expert's answer
2022-03-09T08:28:37-0500

(a) GCD(105,231)

Set up a division problem where a is larger than b.

a ÷ b = c with remainder R. Do the division. Then replace a with b, replace b with R and repeat the division. Continue the process until R = 0.


231 ÷ 105 = 2 R 21    (231 = 2 × 105 + 21)


105 ÷ 21 = 5 R 0    (105 = 5 × 21 + 0)


When remainder R = 0, the GCF is the divisor, b, in the last equation. GCD = 21 [Answer]


n=+f(x)\sum_{n=-\infty}^{+\infty} f(x) \\


(b) Use mathematical induction to show that


1 · 1! + 2 · 2! + · · · + n · n! = (n + 1)! − 1

S1=1.1!=1

=(1+1)!-1=2!-1=2-1=1

Sk= 1.1! + 2 · 2! + · · · + k · k! = (k + 1)! − 1

Sk+1= 1.1! + 2 · 2! + · · · + k+1 · (k+1)! = (k+1 + 1)! − 1=(k+2)!-1


Therefore,

Sk+ Ak+1 =Sk+1, where Ak+1 = k+1 · (k+1)!

(k+1)!-1 +k+1 · (k+1)! = (k+1)![1+(k+1)]−1 = (k+1)![k+2]−1=(k+2)!−1 [Answer]


(c) Show that the relation ∼ defined on R as a ∼ b if b−a ∈ Q, is an equivalence relation.


Also, find the equivalence class of 1.



Let us show that a relation '\sim' on R\mathbb R by 'aba \sim b if aba - b  ∈ Q is an equivalence relation, that is reflexive, symmetric, and transitive.


For each aRa\in\mathbb Raa=0Qa-a=0\in Q , and thus aaa\sim a each aRa\in\mathbb R, and the relation is reflexive.


If aba\sim b, then abQa-b\in Q. It follows that ba=(ab)Qb-a=-(a-b)\in Q, and thus ba.b\sim a. Consequently, the relation is symmetric.


Let aba\sim b and bcb\sim c. Then abQ, bcQa-b\in Q, \ b-c\in Q. It follows that ac=(ab)+(bc)Qa-c=(a-b)+(b-c)\in Q, and thus aca\sim c. Therefore, the relation is transitive.


By definition, the equivalence class of an element aa is [a]={bR  baQ}[a]=\{b\in\mathbb R\ |\ b-a\in Q\}.


Let us give the equivalence classes of 1


[1]={bR  b1Q}={bR  b=1+n, nQ}=1+Q[1]=\{b\in\mathbb R\ |\ b-1\in Q\}\\ =\{b\in\mathbb R\ |\ b=1+n,\ n\in Q\}\\=1+Q [Answer]



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