Explanation
$$ 203 \times 197 = (200 + 3) \times (200 - 3) $$ Applying the formula $$ (a+b)(a-b) = { a }^{ 2 }-{ b }^{ 2 } $$, where $$ a =200 , b = 3 $$$$ 203 \times 197 = (200 + 3) \times (200 - 3) = { 200 }^{ 2 }-{ 3 }^{ 2 } = 40,000 - 9 = 39991 $$
$$ 20.8 \times 19.2 = (20+ 0.8) \times (20 - 0.8) $$ Applying the formula $$ (a+b)(a-b) = { a }^{ 2 }-{ b }^{ 2 } $$, where $$ a =20 , b = 0.8 $$
$$ 20.8 \times 19.2 = (20+ 0.8) \times (20 - 0.8) = { 20 }^{ 2 }-{ 0.8 }^{ 2 } = 400 - 0.64= 399.36 $$
$$ {391}^{2} = {(400 - 9)}^{2} $$It is the form of $$ {(a - b)}^{2} $$, where $$ a = 400, b = 9 $$Applying the formula $$ { (a - b) }^{ 2 } = {a}^{2} + { b }^{ 2 } - 2ab$$$$ {391}^{2} = {(400 - 9)}^{2} = {400}^{2} + { 9 }^{ 2 } -2\times 400 \times 9 = 1,60,000 + 81 - 7200 = 152881 $$
$$607$$ can be written as $$600+7$$
$$\therefore {607}^{2} = {(600 + 7)}^{2} $$It is the form of $$ {(a+b)}^{2} $$, where $$ a = 600, b = 7 $$Applying the formula $$ { (a+b) }^{ 2 } = {a}^{2} + { b }^{ 2 } + 2ab $$$$ {607}^{2} = {(600 + 7)}^{2} = {600}^{2} + { 7 }^{ 2 } +2\times 600 \times 7 = 360000 + 49 + 8400 = 368449 $$
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