If √x + 8 = 7, then √x = -1, so x does not exist, because √ only refers to positive roots.
If √(x + 8) = 7, then x + 8 = 49 and x + 41.
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7*sqrt(2) = 9.899 to 3 dp
5x2 + 8x = 7 5x2 + 8x - 7 = 0 This cannot be factorised so the solutions need to be determined using the quadratic formula The solutions are {-8 ± sqrt[82 - 4*5*(-7)]}/(2*5) = {-8 ± sqrt[64 + 140]}/10 = {-8 ± sqrt[204]}/10 = -2.22829 and 0.62829 (to 5 dp)
144 9+7 = 16 and 9*16 = 144
As written, that is true. (7 - 1) + 8 = 7 - (1 + 8) is not true.