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CBSE Class 9 Answered

Prove square root of 2 space end root space plus space square root of 5

 is a irrational number.
Asked by agraharianiruddha | 20 Jul, 2018, 09:29: PM
answered-by-expert Expert Answer
begin mathsize 16px style Let space square root of 2 space plus space square root of 5 space be space straight a space rational space number.
rightwards double arrow square root of 2 space plus space square root of 5 space equals space straight p over straight q
rightwards double arrow straight p over straight q space minus space square root of 2 space space equals square root of 5
rightwards double arrow open parentheses straight p over straight q space minus space square root of 2 close parentheses squared space equals space 5
rightwards double arrow straight p squared over straight q squared space minus space fraction numerator 2 space square root of 2 over denominator straight q end fraction straight p space plus space 2 space equals space 5
rightwards double arrow straight p squared over straight q squared space minus space 2 space equals space fraction numerator 2 space square root of 2 over denominator straight q end fraction straight p space
rightwards double arrow fraction numerator straight p squared space minus space 2 straight q squared over denominator straight q squared end fraction space equals space fraction numerator 2 straight p over denominator straight q end fraction space square root of 2
rightwards double arrow fraction numerator straight p squared space minus space 2 straight q squared over denominator 2 pq end fraction space equals square root of 2
rightwards double arrow square root of 2 space is space straight a space rationla space number. left parenthesis space since comma space straight p. space straight q space are space integers space rightwards double arrow fraction numerator straight p squared space minus space 2 straight q squared over denominator 2 pq end fraction space is space straight a space rational space numbers right parenthesis
But space thist space contradicts space the space fact space that space square root of 2 space is space an space irrational space number.
So comma space our space assumption space is space wrong.
Hence space square root of 2 space plus space square root of 5 space space is space an space irrational space number.
end style
Answered by Yasmeen Khan | 23 Jul, 2018, 11:41: AM
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