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(a+b)(a+b) = 10\times10=100
a^2 + ba + ab + b^2 = 100
a^2 + b^2 + 2ab = 100
a^2 + b^2 + 2\times21 = 100
a^2 + b^2 + 42 = 100
a^2 + b^2 = 58
(a+b)(a+b) = 10\times10=100a^2 + ba + ab + b^2 = 100a^2 + b^2 + 2ab = 100a^2 + b^2 + 2\times21 = 100a^2 + b^2 + 42 = 100a^2 + b^2 = 58