n=n1n2, n1≥2,n2≥2 .
Use a proof by contradiction. Let us assume that both of n1,n2 are greater than or equal to n1/2.
Case I
n1=n1/2
n2=n1/2
n1n2=n21+21=n
Case II
n1=n21+x;x>0
n2=n21+y;y>0
n1n2=n21+xn21+y;x>0;y>0
n1n2=nnx+y;x>0;y>0
So, x+y=0
x=−y but x>0,y>0
which is a contradiction.
So,If a positive whole number n can be expressed as n1n2 , where n1 is greater than or equal to 2 and n2 is greater than or equal to 2, then at least one element of n1 and n2 is less than n1/2 .
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