[填空题]
Repeat Problem 14 for thurg5bd cap +f6l .rzi.-9r,ow e crc(e8-*ewnb,xq k. zxase when two of the positive charges, on opposite corners, are rxke.re(qc8b- *wnz x,eplaced by negative charges of the same magnitude (Fig. 16–51).
$F_4$ = $ \times10^{ 7}\,N$
$\theta$ = $^{\circ}$
