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let the number need be squared = 10x+5 e.g. 25 => x=2 i.e. (10x+5)² is the number we need then the formula that use to calc. is x(x+1)*100+25 e.g. 25² = 2(2+1)*100+25 = 625 Pf. of (10x+5)²=x(x+1)*100+25 L.H.S. = 100x²+100x+25 R.H.S. = 100x²+100x+25 = L.H.S.