Given
x → 0 lim ( e x − x x 2 + x 6 x − e x ) = p + ln q
for rationals p , q , find the value of p + q .
This is a part of the College Calc problem set. You can find more problems here .
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We can use L'hopital when we combine the fractions! x → lim ( e x − x x 2 + x 6 x − e x ) = x → 0 lim x e x − x 2 x 3 + 6 x e x − e 2 x − x 6 x + x e x = L’H 0 0 x → 0 lim e x + x e x − 2 x 3 x 2 + ln ( 6 e ) 6 x e x − 2 e 2 x − 6 x − ln 6 x 6 x + e x + x e x = ln 6 + 1 − 2 − 1 + 1 = ln 6 − 1
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Note that
x → 0 lim e x − x x 2 = 1 − 0 0 = 0 ;
be careful not to use l'hopital on the limit above. As for the second term,
x → 0 lim x 6 x − e x = x → 0 lim x ( 6 x − 1 ) − ( e x − 1 ) = ln 6 − ln e = − 1 + ln 6 .
Therefore, p = − 1 and q = 6 such that p + q = 5 .