Question

some of all 2 digit number​

Answer 1

a=10 , d =11-10=1, n=9p, Sn=?

wkt:Sn =n/2(2a+(n-1)d)

S90=90/2(2×10+(90-1)1)

S90=45(20+89)

S90=45(109)

S90=4905

therefore the sum of all 2 digit numbers is 4905

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