Probability lies in [0,1]

If p p and q q are chosen uniformly and randomly, with replacement, from the set { 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 } \{ 1,2,3,4,5,6,7,8,9,10 \} . Determine the probability that the roots of the equation x 2 + p . x + q = 0 x^2+p.x+q=0 are real.

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The answer is 0.62.

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1 solution

F i r s t o f a l l , t h e t o t a l n u m b e r o f o u t c o m e s c a n b e e a c h n u m b e r w i t h a l l o t h e r s , i . e . ( 1 , 1 ) ; ( 1 , 2 ) . . . . . ( 10 , 9 ) ; ( 10 , 10 ) w h i c h i s a 100 o u t c o m e s . N o w f o r x 2 + p . x + q = 0 t o h a v e r e a l r o o t s , w e n e e d p 2 4 q S o , b y r e j e c t i n g c a s e s a n d b y r e c o g n i z i n g a p a t t e r n , W e c a n s e e f o r p = 1 , q h a s n o v a l u e . p = 2 , q = 1 p = 3 , q = 1 , 2 p = 4 , q = 1 , 2 , 3 , 4 p = 5 , q = 1 , 2 , 3 , 4 , 5 , 6 p = 6 , q = 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 p = 7 , 8 , 9 , 10 ; q = a l l t h e n u m b e r s . S o t o t a l n u m b e r o f p a i r s p o s s i b l e = 1 + 2 + 4 + 6 + 9 + 10 + 10 + 10 + 10 = 62 S o , p r o b a b i l i t y = 62 100 = 0.62 First\quad of\quad all,\quad the\quad total\quad number\quad of\quad outcomes\quad can\quad be\quad each\quad \\ number\quad with\quad all\quad others,\quad i.e.\quad (1,1);(1,2).....(10,9);(10,10)\quad \\ which\quad is\quad a\quad 100\quad outcomes.\\ \\ Now\quad for\quad { x }^{ 2 }+p.x+q=0\quad to\quad have\quad real\quad roots,\quad we\quad need\quad { p }^{ 2 }\ge 4q\\ So,\quad by\quad rejecting\quad cases\quad and\quad by\quad recognizing\quad a\quad pattern,\quad \\ We\quad can\quad see\quad for\quad p\quad =\quad 1,\quad q\quad has\quad no\quad value.\\ p=2,\quad q=1\\ p=3,\quad q=1,2\\ p=4,\quad q=1,2,3,4\\ p=5,\quad q=1,2,3,4,5,6\\ p=6,\quad q=1,2,3,4,5,6,7,8,9\\ p=7,8,9,10;\quad q=all\quad the\quad numbers.\\ So\quad total\quad number\quad of\quad pairs\quad possible=1+2+4+6+9+10+10+10+10\\ \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad =62\\ So,\quad probability\quad =\quad \frac { 62 }{ 100 } \quad =\quad 0.62

Did the same!

Kartik Sharma - 6 years, 6 months ago

Wow , I rounded to 0.6 for no reason at all

Anand Iyer - 6 years, 6 months ago

carelessly wrote 0.61 but it's true :v

Figel Ilham - 6 years, 6 months ago

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