If the square of (a + 21) = 1512900, then (a + 11) times (a + 31) equals

solution
(A+21)²=1512900
∴A²+42A+441=1512900
∴(A+11)(A+31)
=A²+42A+341
=1512900-100 - (100 less than above)
=1512800

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